Once a Season

A recurring story element is used as a form of tradition within a show each season. This doesn't necessarily mean it is a Recycled Script, as more often than not the only similarity is the subject. The story itself can go in many different directions. At other times it can just be a Day in The Limelight, focusing on a specific character who normally doesn't get that much attention in the regular episodes. A sister trope to Once an Episode.

It should be made very clear that this isn't about a specific Myth Arc that is scattered at different parts of the season. Usually this episode's schedule is very consistent, especially if it is set around Sweeps. In some cases fans may note that there might be a set-up for such a character to appear once a season, only for the consistency to jump around to multiple seasons between appearances.

Much of the time it ends up as a Something Completely Different episode.

Compare Mandatory Line, Good Troi Episode, Once an Episode, Milestone Celebration and any number of the holiday tropes such as Christmas Episode and Halloween Episode.

Anime & Manga

 * Digimon: Once a season you will have at least one main character with a sibling complex:
 * Adventure has Yamato (as well as Taichi), 02 has Ken, Tamers has Jenrya, Frontier has Koji, Savers has Tohma (and Masaru), and Xros Wars has Nene.
 * With one exception, a Leomon (or something in it's evolutionary line) will be a reccuring character and then die Once A Season.
 * Apart from the Orange Islands or Johto, which didn't introduce one, the Pokémon anime had Pikachu fry the bike of Token Girl once per region. In the latest region, Pikachu shocked the Token Girl herself.
 * Ash and crossdressing also goes Once a Season.
 * With the exception of the Orange Islands and Hoenn sagas, a projection of a Legendary Pokémon would appear to Ash and/or his friends at the beginning of that saga (Ho-Oh in Kanto, Suicune in Johto, Mesprit in Sinnoh, Zekrom in Unova).
 * Again, with the exception of the Orange Islands, Ash always ends up losing the Pokémon tournament at the end of the season.
 * Pretty Cure, for its first five seasons, had a number of traditions: infighting in episode 8, new mascot introduced just before the midseason powerup, new mascot runs away from home shortly after, just to name a few. Most of these were done away with along with the Art Shift and the name-based mascot sentence enders come Fresh Pretty Cure.
 * Slayers had one crossdressing episode every season, and it was always episode 17.

Film

 * For a movie format, Star Wars did this purposefully.
 * The Phantom Menace / A New Hope: Young kid pilot managing a lucky shot to destroy a massive battleship.
 * Attack of the Clones / The Empire Strikes Back: A battle in an asteroid field zig-zagging around massive chunks of rock.
 * Revenge of the Sith / Return of the Jedi: A duel between an apprentice and someone being manipulated into becoming the next apprentice.
 * Let's not forget every Star Wars movie contained one Big No. Obi-Wan in I and II, Vader in III, and Luke in IV, V, and VI.
 * Let's not forget that George Lucas is editing in another one from Vader in VI.

Live Action TV
"Giles: We need to save Buffy from Hansel and Gretel! Cordelia: Now, let's be clear, the brain damage happened before I hit you."
 * Bonanza: Starting with Season 12, a crooked businessman named Bradley Meredith (Lorene Greene in a dual role) came to Virginia City to cause trouble, most notably for the Cartwrights and his prime target, Ben. The first installment came in 1971's "A Deck of Aces." The second installment in what was to be an annual storyline, "One Ace Too Many," aired in the spring of 1972. A third installment involving Meredith's latest scheme to cash in on the Cartwright's name, was planned for the spring of 1973, but "Bonanza' was canceled before the script was completed.
 * Buffy the Vampire Slayer had "Buffy's Birthday" episodes, where something always crashed her party. In the third season it seemed like Spike and Ethan Rayne were going to make yearly visits, but Spike became a cast member and Ethan wasn't seen after Season 4.
 * Spike lampshades how something terrible happens on Buffy's birthday in the season six episode.
 * Oh, yeah, and every season has a minimum of one attempt to destroy the world. This would often be the Season Finale, but sometimes it would come in the middle of the season, in which case it was rarely taken that seriously.
 * The producers also commented that they worked hard to give Giles one line per season that sounds absolutely crazy out of context, and to lampshade its craziness. Their personal favorite was from Season 3's episode "Gingerbread":


 * Amy Madison made an appearance in human form once per season, except for season 5 (when she was absent) and season 6 (where she made brief appearances over three episodes).
 * There were also the Halloween episodes, where someone always claims that it's the one night the supernatural takes off. . . and yet some aspect of the supernatural didn't get the memo.
 * In every season finale of Charmed, the last shot is the front door of the Halliwell Manor closing, either being magically closed by Prue (seasons 1 and 2), another supernatural being (seasons 3 and 4), Chris (season 5), itself (season 7) or (season 8). The only exception is season 6, the final shot shows the doors of a hospital room closing.
 * Though in the season 6 finale, Chris does magically close the front door. Only this time it happens during the episode (when he throws Darryl out), and not at the very end.
 * Cheers had the yearly "Bar Wars" episodes, in which Cheers and another local bar (Gary's Old Towne Tavern) competed to top each other in practical jokes.
 * Community so far has had a Halloween and Christmas episode in all three seasons.
 * Seasons 1 and 2 both had episodes centered around Greendale's paintball tournament. Whether or not season 3 will have one remains to be seen.
 * Seeing as seasons 2 and 3 both have episodes where Abed shoots a documentary of the episode's events (Pierce's hospitalization in season 2, The Dean's commercial and mental breakdown in season 3) this may or may not become a yearly tradition as well.
 * Doctor Who has its yearly Christmas episode. There was a tradition starting where it would always snow, but it would always really be something else, like ash from an exploding spaceship, but this was dropped.
 * The revived Doctor Who has a Dalek episode every season (two in the first series), as mandated by the contract that allowed them to use the Daleks.
 * Tradition broken.
 * In addition, between series 2 and 4 we had a yearly episode in which the Doctor was reduced to a cameo commonly nicknamed "Doctor-lites" by the fandom.
 * For the first four years of the revival, the twelfth episode of each season featured a few celebrity guest-stars. The first season made the most of it by having the likes of Anne Robinson voice homicidal robot versions of themselves in a Deadly Game. The next three years just had the celebrities appear on TV, commenting on the ghosts / Mr Saxon's election campaign /
 * It hasn't been explicitly stated as a deliberate tradition, but every year of New Who has had one episode when they go back in time and meet a famous British person and often use the story to fill in a mystery in said famous person's life as a kind of Historical In-Joke:
 * Season 1: Charles Dickens. Dickens planned to change the end of The Mystery of Edwin Drood to include the monsters he saw, but died before he could finish.
 * Season 2: Queen Victoria. They explained the mystery of her hemophilia to being attacked by (and possibly being) a werewolf.
 * Season 3: William Shakespeare. A trio of witches is responsible for his lost play Love's Labours Won (and they try to imply that Martha was Shakespeare's "Dark Lady.")
 * Season 4: Agatha Christie. Her infamous disappearance was due to her memory being blocked after being attacked by a Vespiform (and Donna Noble originally thought up Miss Marple).
 * Season 5: Winston Churchill. One of his weapons to help win the war was going to be his "Ironside" (really a Dalek). Another episode focused on Vincent van Gogh. It turns out that 'Sunflowers' was inspired by Amy putting some sunflowers in Vincent's garden.
 * Season 6: Pirate Captain Henry Avery. His infamous disappearance was caused by his crew being serially abducted by an AI on an abandoned hospital spaceship, and he chooses to stick around and pilot the thing when he finds out staying in the ship is the only way to prolong the life of his terminally ill son.
 * Several of the characters on Frasier make seasonal guest appearances, including his agent Bebe, his ex-wife Lilith, and their son Frederick.
 * Like holiday episodes, the Seabees (Seattle Broadcast Awards) by definition came up only once a year.
 * Doogie Howser, M.D.: Given that Doogie’s young age is such an important factor in the series, his birthday (September 21) was always tied into the plot of every season premiere to pave the way into the next chapter of his life as a teenage doctor.
 * Because Season 4 aired many episodes Out of Order, the initial “Doogie’s birthday” episode for the season was delayed until the next week (“Look Ma, No Pants”), in order to make room for the "Very Special Episode” season premiere, which focused on the 1992 Los Angeles Riots.
 * The “annual father-son fishing trip” episode theme was also repeated, each with a different twist, every season.
 * The Dukes of Hazzard: At different points of the season starting in Season 2, Hughie Hogg (Jeff Altman as Boss Hogg's even more crooked and unethical nephew) made annual appearances. Usually but not always, the plot involved Hogg coming up with a sure-fire scheme to make money for Boss, and Boss becoming wary but going along when Hughie said that he would also find a way to pass the blame onto the Duke boys; however, the scheme always had the ulterior motive of ruining Boss. It would be up to Bo and Luke to find enough evidence to run Hughie and his cronies out of town.
 * After appearing several times during the first three seasons of Friends, Janice still appears once each of the subsequent seasons.
 * House would often towards the end the season (if not the Season Finale) have some sort of hallucination episode, sort of started by the "Three Stories" episode in the first season that had House using hypothetical situations to better understand a few medical dilemmas.
 * House would also have one antagonist per season. Vogler, Tritter and Amber all play this role, as does Stacey's husband..
 * Several seasons also feature an In Medias Res episode near the end of the season (seasons 4's "House's Head", season 6's "Help Me" and season 7's "Moving On").
 * Legend of the Seeker seems to be following the example of the novel series it's based on, with a new Wizard's Rule each season.
 * Lost:
 * In every season premiere, the opening scene is a Tomato Surprise about the character being shown, the character's location, or both:
 * The series itself begins In Medias Res with Jack waking up in the jungle and discovering the plane wreckage
 * Desmond inside the nicely furnished hatch in season 2
 * Juliet and the Others living in a modern town on the island in season 3
 * Hurley being chased by police in season 4
 * with the DHARMA Initiative in 1977 in season 5
 * Jack meeting on Flight 815 and the island being  in season 6
 * In every season finale, something pivotal to that season blows up:
 * The hatch door and the raft in season 1
 * The hatch (Swan station) itself in season 2
 * Many of the Others when they attack the beach in season 3
 * in season 4
 * in season 5
 * in season 6, though that was more of an implosion
 * Additional "once a years":
 * Since season 2, at least one episode each season is one long flashback (usually with a frame story). These are "The Other 48 Days" (season 2), "Flashes Before Your Eyes" (season 3), "Meet Kevin Johnson" (season 4), "316" and "The Life and Death of Jeremy Bentham" (season 5), and "Ab Aeterno", "Happily Ever After" (which flashes sideways instead of back) and "Across the Sea" (which ditches the frame story and features only one starring character, but played by a different actor) (season 6).
 * One member of the original cast has died each season. These are (season 1),  (season 2),  (season 3),  (season 4),  (season 5), and  (season 6).
 * Benjamin Linus gets the snot kicked out of him once each season he appears in. Characters who get the honors of beating his face in are Sayid (season 2), Jack (season 3), Sawyer (season 4), Desmond (season 5), Desmond again (season 6 ). Hilariously,.
 * NCIS: Tony gets framed for a homicide Once a Season. He is Genre Savvy enough to notice and lampshade it, but not enough to prevent it.
 * Gibbs builds and (somehow) disposes of a boat in his basement once a season.
 * Power Rangers has only one instance where a Sixth Ranger doesn't show up in a season (season three, although that includes an entire ranger team stepping in for a Story Arc). They were so dedicated to the Sixth Ranger trope they created the American Titanium Ranger for Power Rangers Lightspeed Rescue when the Super Sentai Go Go V original didn't have one.
 * As well as the Mid-Season Upgrade, the Reunion Show (the traditional team-up began in season 7) and the battlizer (began in season 6). The crossover specials didn't always happen (Ninja Storm and RPM, for complicated production reasons). Super Sentai always had a crossover movie though.
 * The Sopranos tends to have one episode per season to showcase Tony's dream sequences.
 * In every season, even seasons where his storyline is not particularly prominent everything will stop so we can focus on Christopher. Other characters receive episodes featuring them prominently but most of these episodes focus almost soley on Christopher's development or he is at least most central to the storyline.
 * Season 1: "The Legend of Tennessee Moltisanti"
 * Season 2:" D-Girl" and to a certain extent "Full Leather Jacket"
 * Season 3: Fortunate Son
 * Season 4: This one's the debatable but the season premiere and "The Strong Silent Type" are the prime candidates.
 * Season 5: "Long Term Parking"
 * Season 6 part 1: "The Ride"
 * Season 6 part 2: "Walk Like a Man"
 * Stargate SG-1 has Clip Shows. Not quite every single season, but it seemed that way for a while.
 * Star Trek: The Next Generation had yearly visits from Q (Apparently Gene Roddenberry wanted him to appear three or four times a season, but upon a lackluster second appearance they decided on a simple yearly visit). They also had the Holodeck Malfunction, which became a tradition for Deep Space Nine and Voyager as well.
 * TNG episodes about the painfully socially awkward and struggling man Lt. Barclay almost qualify after his introduction in Season 3, but didn't get one in Season 5 Instead, he got two Season 6 episodes. One of those two ("Ship in a Bottle") was also the Season 6 Holodeck Malfunction episode and featured some nice Continuity Nods for both Season 2's Holodeck Malfunction episode ("Elementary, Dear Data", in which a Professor Moriarty hologram becomes self-aware in Data and Geordi's leisure time) and Barclay (who did was not on the Enterprise during the events of Season 2 and unlocked Moriarty's program, not knowing he had been neglectfully been sealed away, and whose introductory episode was about his bad holodeck habits).
 * Deep Space Nine's producers allowed Colm "O'Brien" Meaney time off regularly to maintain his film career, so when they did have full access to him, they regularly put his character through the mill, either by having him serve thirty years for murder (albeit All Just a Dream) or have his daughter disappear into a time rift or whatever (also because he was considered the likeable everyman of the cast, so making him suffer was thought to have more impact for the audience). The "O'Brien Must Suffer" episode became a tradition.
 * Deep Space Nine also did a mostly-comedic "Ferengi episode" about once or twice a season.
 * Several seasons of Deep Space Nine also had one visit to the Mirror Universe which generally featured one of the Mirror Ferengi getting killed.
 * Star Trek: Enterprise had at least one spotlight episode a season for the Andorian Captain Shran and his uneasy friendship with Captain Archer (although he did make a few minor appearances in other episodes). The fourth season, organized into mini-arcs of two-three episodes, had one with Shran in the middle of it.
 * Thirty Rock has Dennis Duffy, self-diagnosed sex addict, former beeper mogul and ex-Subway Hero, appear once a season.
 * Some challenges or challenge types have been repeated in Survivor they become a tradition. Examples include:
 * The Car challenge held later in the game; averted by later seasons though.
 * The "Car curse". Whoever wins a car will not win the game.
 * A survivor auction in place of a reward competition
 * An Eat That type of challenge; but these appear to have stopped after China.
 * Family visits or a Letters From Home challenge
 * The penultimate immunity challenge is sometimes a composite challenge consisting of obstacles from previous challenges in the game, or even past seasons.
 * A challenge surrounding trivia about other Survivor contestants that season or opinions about other contestants.
 * The Torch Walk or Rites of Passage as they are often called before the final immunity challenge, wherein the contestants still in the game (Final four or final three) remember the previously evicted players and then burn the torches down while sad music plays. They also have to go on a hike or boat ride full of Scenery Porn. At the end is always the final immunity challenge.
 * The Final immunity Challenge is often an endurance competition. Often it is either physical endurance or willpower.
 * Subverted in a couple seasons. The final immunity challenge in Australian outback was the trivia (To be fair; this was the second season.) Gabon violated the tradition by having the final immunity challenge be stacking cards. Heroes vs. Villains violated this by having the final immunity challenge be a blind maze run to grab immunity. Redemption Island violated this by having the final immunity challenge be a large maze followed by the fourth puzzle in a row for Rob's sake.
 * The final two or three being given a special breakfast on the last day.
 * The final two or three burning down the camp on the last day.
 * In the American Big Brother:
 * The first head of household competition is almost always endurance or physical competition of some kind. Averted in season 10 where the public voted on the first Head of Household.
 * Majority Rules is commonly played within the first one or two weeks; explainable because the competition works best when there are about ten people.
 * The head of household always receives a letter from home.
 * Sometimes; a movie is shown to winners of a luxury competition.
 * A shuffleboard type challenge is often played at one point.
 * Sometimes, an arcade-like challenge is played.
 * A challenge involving copious amounts of liquid or food is often played at one point.
 * A food/Luxury competition is often ignored whenever the challenge is either an Endurance or a long-task.
 * A veto competition where Otev (Or a previously evicted houseguest) asks houseguests to pick up an object with the answer to their question and bring it to them.
 * A luxury or shopping spree challenge.
 * Double Eliminations
 * A "Before or after" competition.
 * A "True or false" competition similar to that.
 * A visit from someone outside the house.
 * Face morphing challenges
 * The final three Head of Household competition is always played in three parts. The first part is always endurance, the second part is always a puzzle involving the order of houseguests' eliminations or the head of households, the third part is always asking questions about what the jurors would say.
 * 24: Around episode eight, the Big Nasty Thing That's Going On turns out to be a small portion of the Big Bad's overarching plan.
 * And around episode 14, that Big Bad is stopped and revealed in a side plot to be actually either The Dragon of the Bigger Bad / Evil Overlord, or a free or ancillary agent whose operation was actually damaging the Evil Overlord's one.
 * On a more meta level, since as early as Season 1, 24 has managed to predict an important item (usually a failure) of US foreign politics or military operations that actually takes place next year or a couple of years later in real life: the first African-American President in Season 1 (way before its time); the US government blaming terrorist attacks on Middle-East countries to start a war for oil in Season 2; tortured or neglected agents defecting to the other side in Season 3; excessive demonization of arabs in the public media in Season 4; the US getting involved in the (mis)relationship between "mother Russia" and her annexed territories Season 5 (not to count revealing the President as the evil behind actually trying to start a war). Seasons 7 and 8 being the most recent ones it is not clear yet what 24 is going to end having predicted but it may be related to Sarah Palin as the first female President or American firms testing bioweapons in African countries (S7), or (S8).
 * Ever since Season 2, someone important dies at 02:00 AM, or during the episode running that hour:
 * Season 1: former CTU District Manager Richard Walsh hands evidence to Jack and dies right before the hour (setting part of the tone of the series).
 * Season 2: Jonathan Wallace (the villain who carried the Cyprus Audio). Also, Jack "dies" at the end of the hour.
 * Season 3: Nina Myers dies at the end of the hour, at the hands of Jack.
 * Season 4: both Paul Raines and a witness brought by Jack from the Chinese consulate.
 * Season 5: the inversion: former Secretary of Defense James Heller is confirmed to have survived an attack that took place near the end of the previous hour.
 * Season 6: Milo Pressman, feigning being the director of CTU to protect its personnel from a raid, dies during the hour.
 * Season 7: Tony Almeida killed Larry Moss shortly before the hour, and kills his own accomplice at the end of the hour.
 * Season 8: a kid sent by the terrorists to kill a loose end is remotely detonated near the end of the hour. Also, the villain who appeared to mastermind the events up to that point, and who the terrorists were trying to kill, is confirmed to have been already dead during the hour (part of Jack's ploy to draw the terrorists to the open).
 * The OC had the annual Christmukkah episode.
 * Scrubs has the episodes where the narration shifted to another character, titled, in each instance, either "His Story" or "Her Story," or, on one occassion where minor characters were used, "Their Story."
 * There was also usually at least one episode with the characters singing a song in one of J.D.'s Imagine Spots (the show often had elaborate musical montages and characters singing in-character, but this was more like a full broadway musical). This culminated into a full Musical Episode where a patient with a brain tumor could only hear them singing.
 * Gossip Girl has thus far had at least one wedding each season.
 * The last scenes of the season finales always take place a week after the prior events in the episode.
 * Episodes seven and eight of the season always have significant events for Chuck and Blair.
 * Titus started a tradition of having someone else narrate from the neutral space, with Papa Titus taking it for an episode in season two and Erin taking an episode in season three. If they had more seasons they would have continued with Dave and Tommy.
 * The Big Bang Theory has at least one episode a season where the guys dress up in costumes, sometimes coordinated costumes as well (and no, it's not always a Halloween Episode). It's enough of a tradition that an image of the guys is usually a centerpiece on the DVD inserts. The most epic one has to be where they all dress up (including Penny and her current boyfriend Zach) as the Justice League of America for a New Years party.
 * There is also so far one episode a season that includes Penny and Sheldon singing "Soft Kitty," a song his mom would sing for him when he was sick as a child. In the fifth season his Mom sang it to him herself.
 * Starting in the third season there is at least one appearance by Wil Wheaton and Sheldon's rivalry with him.
 * Early in the first season Sheldon's mom Mary came to visit, and mid second season Leonard's mom Beverly did the same. While it has sometimes been sporatic (Mary didn't show up in the second season and Beverly in the fourth) both have made fairly regular appearances.
 * Red Dwarf has some form of time travel once every season.
 * The Coaches' Challenge in The Ultimate Fighter, in which the opposing coaches go head-to-head for $10,000 by playing a different game each season, such as bowling or table hockey. The only season this didn't happen was Season 4 "The Comeback," which did not have team coaches.
 * With the exception of Season 4, Breaking Bad's Wendy has played a small but crucial role once every season.
 * Two Pints of Lager and a Packet of Crisps features the "f-word" only once in each series, always in the series finale.
 * In every season of Insomniac with Dave Attell, Dave would visit New York City for the season finale, and end with returning to his apartment.
 * Wheel of Fortune has certain theme weeks that it does every season, such as Big Money Week, Going Green Week, a college week, a Best Friends week, etc.

Newspaper Comics

 * The newspaper comic Curtis is renowned for its bizarre and over-the-top Kwanzaa stories, which are often the most entertaining comics he does.
 * Garfield always had birthday strips, mostly about how much Garfield dreads them.
 * Several recurring jokes on Peanuts occured once a year. These included Lucy pulling away the football when Charlie Brown tries to kick it, Linus awaiting the Great Pumpkin, and Schroder celebrating Beethoven's birthday. There were also strips commemorating D-Day, and Snoopy going to drink root beers with Bill Maudlin every Memorial Day (based on Charles Schultz's real-life ritual with Maudlin, a cartoonist famous for his satirical cartoons on military life during WWII).
 * About once a year, Beetle Bailey does the gag where the officers receives a written order from the general, with one obvious spelling error that changes the meaning completely. It always end with the officers following out the order exactly the way it was written, because they'd rather look like idiots than to tell the general that he did a mistake.

Web Comics

 * In the webcomic Ozy and Millie, there is a story arc that occurs once a year in which Ozy somehow loses all his fur. This is explained to be because of a family curse that only effects Ozy because he's the only member in the family to actually have fur. It is later explained that the whole curse story was a lie told to him by his father. It Makes Sense in Context.

Western Animation

 * On Codename: Kids Next Door there is a CAKED episode every season.
 * Also, each season had an "art episode". Season 5 didn't have one, but the following season had two.
 * The Simpsons and their annual Treehouse of Horror episodes, starting season 2.
 * Sideshow Bob comes to commit some crime.
 * American Dad! has a non-canonical Christmas episode every season, as well as non-canonical episodes parodying a film or tv show. Most episodes have the main story focus on Stan or Roger, but there are always a few per season that focus on Steve or Francine as well.
 * Each season of Teen Titans had one episode where things would go even zanier than normal. While most episodes had a somewhat serious plotline with a strong focus on characterization, these episodes would just toss all that aside and throw in as much surreal, nonsensical humor as they could.
 * Season 4 had two of these types of episodes.
 * One person dies per season in Avatar: The Last Airbender.
 * After it's second revival (it's 4th season), Family Guy started to do this with the giant chicken fight gag from the Y 2 K episode in season 1.
 * Star Wars:The Clone Wars As of Season 4 Ahsoka Tano had one Designated Girl Fight in each season:
 * My Little Pony Friendship Is Magic: The Elements of Harmony are used Once a Season.