Fictional Holiday

Are you bored with the usual holidays? How about celebrating a new holiday!

This trope deals with holidays non-existent in the real world, in other words holidays that only exist in a story's universe.

Just about any fictional fantasy setting with their own fantasy nations/history/cultures will come with their own fictional holidays. See also You Mean "Xmas", Alternative Calendar.

Anime and Manga

 * Tenchi Muyo! has the Juraian holiday Startica, which happens to involve Christmas trees for some reason.
 * "Heaven's Day" in The Big O, which was the day Paradigm City was founded. It's clearly related to Christmas (giving presents, trees, etc.)

Comic Books

 * An issue of the ALF comic book revealed that Melmac had a variety of holidays.
 * Shown inside are:
 * Eat Off The Floor Day, in which celebrants (are supposed to) dress in bathing suits and eat "duck delight" while sitting on the floor, celebrating the discovery of linoleum by Alexander Graham Cracker. The foodstuff is not made of duck, but makes those who eat it think they're ducks. The two adult Tanners sample it early and go quackers.
 * Wear A Plant Day, in which a tree is planted that is supposed to grow to about a meter in height by evening. It gets nixed immediately by Willie, who discards the seed. A chance raccoon, however, knocks over the can, planting the seed... which begins to grow at an astonishing rate, due to Earth's relatively rich soil. Within the hour, it threatens to start a forest in the backyard that will spread from there in California to the Mississippi River by Thursday.
 * Shown on the front cover and mentioned within are: Bathe With A Policeman Day, Goomer's Birthday, UPC Symbol Day, Eat Your Calendar Day (which appears to be when cover was made, judging by Gordon's activities), Sit On A Friend Day, Play With Your Spaghetti Day, Give A Present To A Wall Day, Wear A Banana Split Day, Yad Sdrawkcab Gnihtyreve Lleps... and Make Up Your Own Holiday Day.
 * In Elf Quest, the desert-dwelling Sun Folk celebrate the Festival of Flood and Flower.

Fanfic

 * Undocumented Features gave us such delightful Dantrovian holidays as "The Festival of the Fudge Sauce", "Terzayyl", and "The Day The Colonists From Earth Finally Understood."

Film

 * Vivian Stanshall, Sir Henry at Rawlinson End (album and film versions): "To celebrate All Squids' Day there was a face-jumping competition at the Fool and Bladder."

Literature

 * As well as Hogswatchnight, Discworld has the Soul Cake Days, which are a sort of cross between Hallowe'en (festivities include trickle-treating and sticky toffee rat on a stick) and Easter (the wizards used to roll boiled eggs down the Tump, and the Soul Cake Tueday Duck lays chocolate eggs). Unfortunately for the Soul Cake Tuesday Duck, it also marks the start of the duck-hunting season.
 * Readers of the diaries and calendars will discover a vast range of other Discworld holidays, ranging from the Creator's Birthday and Patrician Day (28 April and 6th Grune respectively; see also the birthdates of Terry Pratchett and Stephen Briggs) to Treacle Pie Day (June 24, commemorating the burning of a Patrician who imposed an unpopular tax on washing lines). Some of them are only celebrated by the guild or organisation responsible for a particular diary; the Guild of Thieves celebrates Finder's Day, when the ancient rule of "finders, keepers" is in effect.
 * Stephen Briggs was born in a month which doesn't exist?
 * Yobta’ Yupma’ in Star Trek: Klingon Empire, AKA the harvest festival. It’s mostly observed by farmers and natives of farming worlds, and isn’t widely known or celebrated by the rest of the Klingon Empire. Then there's the Andorian Spring Water Festival, as seen in the Star Trek Deep Space Nine Relaunch.
 * P.C. Hodgell's Chronicles of the Kencyrath has, in addition to celebrating the turns of the seasons and solstices, an intercalary day that does not appear on calendars. This is called the Feast of Fools in Tai-tastigon, and is a holiday of lawlessness and excess; in the Kencyr military academy, Tentir, this is the Day of Misrule, where the normal rules of the college do not apply and anyone who snatches the scarf from around a cadet's or officer's neck can command them. In addition, the Tastigon celebrate, or more precisely fear, the Feast of Dead Gods, a night immediately after Autumn's Eve where gods that have lost their followers walk the streets, searching for prey or other things they desire.

Live-Action TV

 * Quark: Happy Number 11!
 * Dinosaurs had "Refrigerator Day", an expy/parody of Christmas. It has elements of Thanksgiving and Hanukkah.
 * Life Day in The Star Wars Holiday Special.
 * And sometimes in the rest of the Star Wars Expanded Universe, a sort of Discontinuity Nod. Mentioned in the X Wing Series.
 * Colonial Day in the re-imagined Battlestar Galactica.
 * Festivus in Seinfeld.
 * Actually, Festivus is real! Come on, everyone, let's decorate the pole! We're done!
 * Let's start with the airing of grievances!
 * The Day of the Dead (Brakiri holiday, not Mexican) in Babylon 5, is observed only once every 200 years.
 * Rose Nylund from The Golden Girls reminisces about St. Olaf holidays: Everybody Hide the Corn Day, Hay Day, the Day of the Princess Pig, and the Day of the Wheat, when everybody came to town dressed as sandwiches.
 * Star Trek: The Original Series episode "The Return of the Archons". The planet Beta 3 had the holiday called Festival, in which the population went totally berserk (destroying, raping, etc.) for 1 hour (the "Red Hour"), then returned to their normal placid demeanor. This was due to their mental conditioning by the computer Landru.
 * Bobunk in Sabrina the Teenage Witch. It's too bad that Salem accidentally deleted it...
 * The pilot of Firefly opens on "Unification Day," a yearly celebration of the Alliance's victory during the civil war some years previous.
 * The Legend of Dick and Dom has Garlic Tuesday, a dey to celebrate all that is smelly.
 * The Thirty Rock episode "Leap Day" transforms leap day (February 29) from non-holiday into full-blown, commercialized, Christmas-esque bash with established traditions and traditional holiday characters. Liz is the only character who had never previously known Leap Day was such a big deal.

MMORP Gs

 * Kingdom of Loathing uses a mixture of real-world and fictional holidays, to go with its fictional calendar: The Festival of Jarlsberg (New Years Day), St. Sneaky Pete's Day (St. Patrick's Day), Oyster Egg Day (Easter), Generic Summer Holiday (Summer Break, more or less, rather than any specific day), Dependence Day (Fourth of July aka Independence Day), and Crimbo (Christmas) circulate around the calendar along with real-world holidays like Valentines Day and Halloween. Borderline cases such as Labór Day and Arrrrbor Day also exist.
 * CAPSLOCK DAY in Billy vs. SNAKEMAN

Music

 * Weasel Stomping Day, a holiday created (in a sense) by "Weird Al" Yankovic in a song detailing a holiday held where people spend the day using large boots to crush weasels.
 * Every March 4th (or the nearest Saturday to it), Doctor Steel's Army of Toy Soldiers celebrate "Toy Soldier Day" (usually by going to the nearest Disney park...)

Newspaper Comics

 * Sadie Hawkins Day in Li'l Abner.

Radio

 * Round the Horne announced some sort of festival every episode—Immerse An Orangutan In Porridge Week, for example. This would be celebrated by equally peculiar events, like floodlit horse massage (at the municipal swimming baths, Tooting).

Tabletop Games

 * Traveller Classic. The Imperium in general had the Emperor's Birthday. One planet had a three day holiday during which the local law level was reduced to zero (i.e. no laws at all) to let the population blow off steam.

Video Games

 * Animal Crossing has a bunch of holidays that are either made-up (Yay Day, La-Di-Day, Bright Lights, Festivale) or Suspiciously Similar Substitute holidays (Toy Day [Christmas], Bunny Day [Easter], etc).
 * "Team Fortress 2" has Australian Christmas. Every December the 17th, children in Australia make weapons and hats for Old Nick, a crotchety old man who lives at the South Pole. After receiving all of his gifts, Old Nick realizes he received many duplicates of gifts, so he sells them off for incredibly low prices!

Web Animation

 * Homestar Runner likes to refer to Christmas as Decemberween.

Web Comics

 * In The Adventures of Dr. McNinja, Dr. McNinja and his family celebrate "Katanaka", which commemorates their Irish ancestors' original victory over pirates. It's observed on Thanksgiving so they can get off work too.
 * In Monsterful we get the "Monster unity day" instead of Halloween, a day where peace and unity of all monsters is celebrated by dressing as other monster species, which is quite normal considering the setting of the comic is a monster-only world.
 * In Triquetra Cats, the Antreyki celebrate Go, a harvest festival style holiday that combines elements of Halloween, Thanksgiving, and Christmas. The main symbol of the holiday is a pumpkin carved with the family symbol topped with an evergreen wreath and harvest fruits a la a cornucopia.
 * Antihero for Hire has Coffee Shops are Closed Day. Naturally, coffee shops are closed on that day.

Western Animation

 * Whacking Day in The Simpsons.
 * They used to also have Do What You Feel Day (formally Do As We Say Day).
 * Clam Day in Family Guy.
 * An episode of Daria had the eponymous character run into anthropomorphic holidays - including Valentines Day, St. Paddy's Day, Halloween, and Guy Fawkes Day. As it was primarily an American (MTV, at that) series, most viewers probably thought the last one was fictional. It's not.
 * Or is it, as Britons generally recognize Bonfire Night?
 * Clearance Day in Sheep in The Big City.
 * In SpongeBob SquarePants, No Spongebob Day.
 * Adding to the Spongebob thing, Leif Erikson Day. Also the Narrator mentions that every day for Spongebob is a holiday. (Even if he has to make one up - though not in the case of Leif Erikson Day, which is a real holiday).
 * Fairly Oddparents. Timmy's alien friend celebrates FLARG, which culminates in him blowing up Earth.
 * Well, it's supposed to be the host planet.
 * Freedom Day in Futurama!
 * In American Dad, Steve tells Stan and Francine that he saw Roger heading into Stan's study with a watermelon and a giant hammer. Francine, thinking Roger has a good explanation, asks Stan if it's Gallagher Day. Stan angrily asks her "do you see a Gallagher Tree, Francine?!"

Real Life

 * The calendar is littered with official days declared by the government, but you don't get to stay home. One that's been in the news recently has been National Nude Hiking Day, which coincided with Governor Sanford's purported hike down the Appalachian trail. I bet every day mentioned on this page will have been silently codified by a fan in some city council somewhere.
 * Talk Like a Pirate Day.

Anime and Manga

 * In Doraemon, Nobita uses one of Doraemon's magical gadgets to declare "Do Nothing Day". Hilarity Ensues.

Film
"Deputy: (suspiciously) What holiday? Allen: Did you ever hear of St. Abernathy's Day? Deputy: No. Allen: That's what we're here for."
 * Woody Allen's film Take the Money and Run has his character escaping from a prison chain gang along with several fellow prisoners to whom he's still chained. At one point they force their way in to an old woman's house to take some food and civilian clothes, and while they're there a sheriff's deputy drops by. Allen's character tells the deputy that he and the others are the woman's cousins, visiting her for "the holiday".

Live-Action TV
"Kenneth: Tonight is the Verdukian Holiday of Mouth Pleasures. Misters Rossitano, Spurloc, and Lutz must have three sausage pizzas followed by some flossing performed by a blonde virgin."
 * In The OC, Seth's family celebrates "Christmukah," a combination of Christmas and Hannukah.
 * Lexx: When Stanley Tweedle blows up the nearest planet, an obstinate brothel satellite manager suddenly "remembers" that he offers free samples on "Bob's Comet Day."
 * In the Thirty Rock episode "Secret Santa," several of the show's staff invent the religion Verdukianism in order to get out of participating in Kenneth's Secret Santa exchange, and when Kenneth buys it, proceed to milk it for all it's worth:

"Andy: Those aren't what you'd call legally recognized holidays."
 * In The Andy Griffith Show episode "Alcohol and Old Lace," two sweet old ladies who are operating a still (in a dry county) excuse themselves by explaining that the moonshine they sell people is meant not for "guzzling," but for "celebrating." Some of the holidays their customers suggest they may be celebrating include Sir Walter Raleigh Landing Day, Muhammed's birthday (observant Muslims are not supposed to drink alcohol), and National Potato Week.

Newspaper Comics

 * In Peanuts, Schroeder didn't make up Beethoven's birthday per se, but he decided to turn it into an observance. It's December 16, by the way.

Web Comics

 * The webcomic Ctrl+Alt+Del has Winter-een-mas, the January holiday invented by Ethan to celebrate video games. It turned into a surprise hit and even led to a brief kidnapping plot when a corporation wanted to take advantage of its popularity.
 * Winter-een-mas is even celebrated in Real Life by some fans.

Western Animation
"Skinner: You fell for our sting, Bart! There's no such thing as Scotchtoberfest! Willie: (distraught) There isn't?!? Ya used me, Skinner! Ya used me!!!"
 * Doug has a subversion to it. Doug imagines his alter ego Quail-Man fighting a villain who wants to get rid of the weekdays. In the end, not only does he foil his plans, but creates a whole new weekday known as Funday in the process.
 * This was accomplished by slowing the Earth's rotation, which somehow made the week eight days long instead of days longer.
 * In Futurama, Bender made up Robanuka as an excuse for not working. The characters later decide to actually celebrate it - by "doing the robot dance", with Jewish music playing in the background.
 * The episode also mentions before that he made up Robanza.
 * The non-canon "Futurama Holdiay Spectacular" episode also features Robanuka as one of the three main holidays celebrated along with X-Mas and Kwanaza. Of course, given what is established above, and the fact that the episode was not canon it doesn't change anything about the holiday being fictional.
 * The Simpsons: When Homer makes up his own religion, he calls into work saying he can't come in because he's celebrating the "Feast of Maximum Occupancy" (he was in Moe's and read it off of a sign.)
 * Skinner made up "Scotchtoberfest" as part of a sting to catch Bart in the act of breaking the rules. Finding out that it was in fact not a real holiday did not sit well with Groundskeeper Willie.