Significant Birth Date

"If the birthday of a character is included in a work, then it is either plot-relevant or significant for the character. Or both."

In Real Life, people are born on random dates. In fiction, their date of birth tends to be deliberately chosen for its symbolic meaning - such as February 29th. The meaning may be straightforward or ironic. It can also, owing to When the Planets Align, have actual meaning.

May lead to an I Remember Because. A specific variation of Significant Reference Date.

Anime and Manga

 * Aoi from Ai Yori Aoshi was born on the day of Tanabata, which celebrates the once-a-year meeting of Orihime (Vega) and Hikoboshi (Altair) across the Milky Way. It's the Asian equivalent to Valentine's Day.
 * xxxHolic
 * Watanuki in was born on April 1st, a date with significance in both the East and the West. It's also a date personally significant to CLAMP; they consider it the anniversary of their founding. Sakura Kinomoto of Cardcaptor Sakura was also born on April 1st, which is a pretty important coincidence... The main character Sakura of Tsubasa Chronicle also has the same birthday.
 * Her brother was born on February 29th, meaning the poor guy could only have a birthday every leap year.
 * Ouran High School Host Club does this. Honey, who is older than he looks, was born on February 29, the joke being that he ages only every four years. The twins Hikaru and Kaoru were born on June 9 (6/9), showing that they're alike, but different. Or, you know...
 * One Piece:
 * Luffy's birthday is on Children's Day (May 5), Usopp's is April Fool's Day, and Chopper's is Christmas Eve. Sanji's, Franky's, and Brook's are word puns. Zoro's birthday is also a word pun, but it falls on Armistice Day.
 * Nearly every major character has a birthday like this, and almost all were chosen by the fans -- Oda just said "Sure Why Not."
 * Ojamajo Doremi: Hazuki Fujiwara's birthday is on Valentine's Day, and this plays a role in a "Naisho" episode.
 * In Axis Powers Hetalia, character birthdays are often a country's Independence Day. This may be because said characters are personified countries.
 * On Urusei Yatsura, the absurdly unlucky protagonist Ataru Moroboshi was born on April 13th, the second-unluckiest day of the year in the Japanese calendar, and in the year he was born was also the anniversary of the Buddha's death.
 * Based on a sample of about 1800 anime and manga characters (in 2004), the most overrepresented date is February 29, occuring about eight times as often as random chance would predict. Second is March 3 (7), followed by May 5 (5), January 1 (4.6), April 1 (4.4), December 24 (4.1), July 7 (4), June 6 (3.8), August 8/October 10 (3.6), and April 4/December 25 (3).

February 29 is Leap Day. March 3 is Girls Day (Hina Matsuri). May 5 is Children's Day. January 1 is New Years Day. April 1 is April Fool's Day and the start of Miyako Odori. December 24 is Christmas Eve. July 7 is Tanabata. June 6, August 8, and October 10 are days that can be written as X/X (6/6, 8/8, and 10/10). April 4 is written 4/4, and 4 is very unlucky in Chinese and Japanese culture. December 25 is Christmas Day.
 * Death Note: What could be a better day for a sugar loving detective to be born than Halloween, the day random strangers give candy to whoever knocks on their door, and being anonymous is perfectly normal?
 * There's also Misa, whose birthday falls on Christmas. Quite appropriate for a character who's childlike, cute and willing to give of herself to others (specifically, Light).
 * A variation/subversion: In Higashi no Eden: Takizawa Akira and Morimi Saki were born one day apart, in the last week of the Showa era. Subverted in that Takizawa Akira is an alias that he picked specifically because of the similar birthdays.
 * In Bakuman｡, Azuki's birthday is November 5. This has absolutely no bearing on the plot of Bakuman｡, but it seems to be the creators' olive branch to the month of November after Death Note, which had particularly nasty developments occur in all three Novembers featured in the series, and especially November 5.
 * In The Cherry Project, Chieri and her love interest share the same birthday.
 * All four main characters of Weiss Kreuz have significant birthdays: Youji's is March 3, which is Girls' Day in Japan; Aya's is July 4, American Independence Day, and he lives in the U.S. for a couple of years, plus his Evil Counterpart is American; Ken's December 23, the Emperor's Birthday and the day before Christmas Eve, he's a lapsed Catholic; and Omi is the ever-popular February 29. Interestingly, it's been deduced that the WK anime takes place in 1998. Omi's canon age is 17. Count backwards and you realize that Omi's year of birth wasn't actually a leap year. Of course, Omi Tsukiyono is a made up identity and doesn't technically exist; why should his birthday?
 * For Slam Dunk, super rookie Rukawa is born on New Year's Day (1/1), The Ace Sendoh on Valentine's Day and protagonist Sakuragi on April Fool's.
 * Interesting variation in Bleach: Rangiku Matsumoto did not have a birthday until her childhood friend Gin found out and made the day they met her birthday. CMOH for many. "Wait, what, that guy?!" moment for everyone else.
 * In Lucky Star:
 * Akira Kogami: Valentine's day, but not much is made of it.
 * Hiyori's birthday is May 24th; Konata's is the 28th. This proximity gives Hiyori a greater sense of them being kindred spirits. (No word on how Konata perceives it.)
 * The Hiiragi twins' birthday is on Tanabata.
 * Misao's birthday is July 20th, which is the original fixed date of Marine Day (Umi no Hi), formerly Marine Memorial Day (Umi no kinen bi), and still falls on it when the 1st of July is a Wednesday. Hiyori says the day predicts a high-energy life.
 * School Nurse Fuyuki Amahara's is September 17th, which is Respect for the Aged Day (Keiro no Hi) when the 1st falls on a Saturday.
 * In Detective Conan, Shinichi's birthday is 4th of May, the day when Sherlock Holmes and Dr. Moriaty supposedly fell of the Reisenbach falls.
 * THE iDOLM@STER - Yukiho's birthday is on the Christmas Eve.

Comic Books
"Alberto Falcone: [To his dad, Carmine] Do you even know when my birthday is? I'll give you a hint. It falls on a holiday."
 * From Batman: The Long Halloween:


 * In Liberality for All, one of the main characters was born on September 11, 2001.
 * Before the Crisis, both Superman and Captain Marvel celebrated their birthdays on February 29th, the leap year day. Certainly the most memorable Superman birthday story was Alan Moore's "For The Man Who Has Everything."
 * Jenny Sparks (from The Authority) was the spirit of the 20th Century so obviously was born on January 1st 1900. In Planetary it was revealed that this was the birthdate of a raft of superhumans designed to act as humanity's "immune system". The Authority also subsequently featured Jenny Quantum, the spirit of the 21st century, who was born... well, you get the idea.
 * Steve Rogers, the original Captain America (comics), was born July 4, 1917.

Film

 * Born on the Fourth of July is pretty self-explanatory.
 * A Very Long Engagement: The main character is a young woman who was born on 1 January 1900.
 * In the song and movie Yankee Doodle Dandy, "Little Johnny Jones" (as played by George M. Cohan [as played by Jimmy Cagney]) claimed to have been born on the 4th of July. Cohan himself was probably born on the 3rd.
 * In The Omen, Damien is born on 6 June at 6 a.m. Think about that a second.
 * In the 1970 Disney film Boatnicks, the main character, a clumsy Coast Guard ensign, was born on December 7, 1941. On Pearl Harbor.
 * Provides an in-joke in Trail of The Pink Panther: We learn that Chief Inspector Jacques Clouseau's birthday fell on September 8. This was the birthday of the role's creator, Peter Sellers.
 * In Friday the 13 th film series, the titular unlucky date is its Villain Protagonist Jason Voorhees' birthday.
 * In Can't Buy Me Love, nerdy protagonist Ronald says he was born on July 20th, 1969 -- the day of the Apollo 11 moon landing.
 * The Bourne Series: Jason Bourne is told he was born on 4/15/71. It's a code, the character telling him that is passing on vital info.
 * In Raintree County, John and Susanna's son Jimmy is born the day the Civil War begins.
 * In The Curious Case of Benjamin Button, the titular character was born on the same day WWI ended.
 * In Forrest Gump, Jenny was born on July 16, 1945, the date of the Trinity Test when the United States successfully tested an atomic bomb in the New Mexico desert.
 * In the first Final Destination movie, the main character's birthday is the 25th September. It coincides with the departure time (9:25) and his designated seat (25 I) of the ill-fated flight he was supposed to be on.
 * Jack, a boy who suffers a unique form of Rapid Aging, was born (prematurely) on Halloween.
 * According to Word of God, the character Elsa was born, which explains her ability to control cold and ice.

Literature

 * Salman Rushdie's novel Midnight's Children is about a man who was born on the exact moment of Indian independence -- 15 August 1947 at midnight -- and who realizes he can communicate telepathically with all the other people in India who were also born on that date.
 * In Something Wicked This Way Comes, the two main characters were born on Halloween and the day after. The day after being All Saints' Day.
 * Harry Potter
 * The Weasley twins -- who are known for being practical jokers -- were born on April Fool's Day.
 * Voldemort was born on New Year's Eve.
 * Harry Potter was born on author J.K. Rowling's birthday, July 31st. And there was a prophecy in book 5 that The Messiah would be born as the seventh month closes. It's stated that it could also have referred to Neville -- born on July 30th.
 * And most importantly, Hermione Granger was born on September 19 -- International Talk Like a Pirate Day.
 * The Curious Case of Benjamin Button: Benjamin's birthday was the day World War I ended.
 * The main character of The Eight by Katherine Neville was born on April 4, which ties in with a lot of the book's numerology and is also said to be an Islamic holy day.
 * In The Dark Is Rising, Will was born on the winter solstice.
 * The Dresden Files:
 * Harry Dresden was born on Halloween. His mother deliberately had him around that time so that he'd be born with the ability to affect Outsiders.
 * Horny Devil Thomas Raith was born on Valentine's Day.
 * In the Circle of Magic books by Tamora Pierce, plant mage Rosethorn was born on Long Night, the midwinter solstice. Ironic because she's a plant mage, and her leafy friends are mostly all asleep on her birthday. She prefers to celebrate at the midsummer solstice instead for this very reason.
 * In the Horatio Hornblower books, or at least the ones published later, Hornblower was born on July 4, 1776, something which he is occasionally teased about (it otherwise never becomes important). It is worth noting that Hornblower was written to be a man of 20th century sensibilities who is fighting in the Wars of French Revolution and the Napoleonic Wars, and thus has some opinions that are regarded as a touch "Republicist" by those who he has offended (or who just feel like needling the nervous skinny guy for fun). Mind you, the author, C. S. Forester, an Englishman though born in Egypt, eventually moved to California to stay, and was likely aiming the books at an American audience. Making Hornblower the same age as the U. S. -- and why not exactly the same age? -- helps fix his place in history in the readers' minds.
 * Lolita: Dolores Haze was born on January 1st, 1935 -- and
 * James Bond nemesis Ernst Starvo Blofeld was born on May 28, 1908. So was Bond creater Ian Fleming.
 * Misery: Annie was born on April Fool's Day.

Live-Action TV
"River: It's my birthday."
 * The Colbert Report: The eagle named after Stephen was born on Patriots' Day. Stephen hopes that this will be a significant birthday and that Stephen jr. will grow up to become the first raptor president.
 * Doctor Who, "A Good Man Goes to War":


 * Admit it, you went back to this moment when River's identity was revealed.

Music

 * 'Levon', according to Elton John, was 'born to a pauper of a pawn on a Christmas day'. The song is from 1971. Move forward to 2010, and guess who has a son on a Christmas day? Zachary Jackson Levon Furnish-John. It's unknown if anyone was suggesting that the name be the first one.

Newspaper Comics

 * In Snake Tales, the Butt Monkey Snake's birthday is April 25, Anzac Day.

Role Playing Games

 * In the Spirit of the Century tabletop role playing game, all player characters were born on 1 January 1901, the first day of the new century.
 * The roleplaying game Honorable Hogwarts adds significant birthdays for several characters, including Valentine's Day for a ladies' man, November 5th for a Rebellious Spirit, and Earth Day for another political-activist character. The Brazilian Independence Day even comes into play.

Tabletop Games

 * In Magic: The Gathering, Urza was born on the first day of the year and Mishra was born on the last. This meant that Urza was one year older than Mishra except on Mishra's birthday.

Theater

 * The Pirates of Penzance features the character Frederic who was born on February 29th, a day that only occurs on leap years. Hilarity Ensues.

Video Games

 * In Metal Gear Solid, Otacon's father was born on the day of the Hiroshima bombing. Compounded by his grandfather having been part of the Manhattan Project. God is not subtle in the MGS-verse.
 * One adaptation of the Sonic the Hedgehog origins had Dr. Robotnik born on the 6th of June, 2006 - 06/06/06.
 * A in-universe subversion of this trope (because the actual date of birth is never revealed in-game): "...born on a certain day, to uncertain parents..."
 * Blaz Blue likes these. The most obvious would be Noel, who as you might guess from her name, was born on Christmas . Many other characters have double dates (i.e. the same day and month).
 * Then Continuum Shift Extend begin with a cutscene who shown Mu-12
 * The Player Character of Fallout 3 was born on 7/13/2258, a reference to The Bible verse Micah 7:13, "The earth will become desolate because of its inhabitants, as the result of their deeds." It sums up the Fallout universe nicely.

Web Comics

 * The protagonist of Homestuck was born on April 13th, which would be a fairly ordinary day... if the numbers 4/13 didn't keep popping up everywhere. His friends are partial aversions, as they were all born in December... on the 1st, 3rd, and 4th.
 * In Eerie Cuties Nina doesn't drink blood like normal vampires, but craves chocolate instead. The only explanation she and her sister got was that it's because she was born on Easter.

Web Original

 * In Arcana Magi Zero, Alysia Perez and Megumi Miyazaki were both born on February 29th, 2000, which made Megumi wonder if she and Alysia shared a common greater destiny as Guardians of the Gate to the Shrine of Four Mythic Elements.

Western Animation

 * In the South Park episode "Awesom-O", Butters' birthday is revealed to be the 11th of September.
 * Roger Ramjet was born on April 1st; April Fools' Day.
 * In The Simpsons, Bart's birthday is also April 1st. While Marge's is April 15th, "same as Hitler" as Homer remembers it. Not quite right.
 * In Hey Arnold!, Eugene was born on Friday the 13th, which exact on isn't mention though.

Real Life

 * Metal guitar virtuoso Steve Vai was born on June 6, 1960 (6-6-60). May explain where he got his near-otherworldly skills...
 * Princess Beatrice of York (granddaughter of Queen Elizabeth II) was born on August 8, 1988 (8-8-88).
 * Ron Kovic was indeed born on July 4, 1946.
 * Muppeteer Steve Whitmire, the current performer of Kermit the Frog, was born on September 24, 1959 -- which happened to be Jim Henson’s 23rd birthday.
 * Physicist Stephen Hawking was born on January 6, 1942 - Three hundred years to the day after the death of astronomer Galileo Galilei.
 * Squick to anyone born exactly 9 months after a parent's birthday.
 * The founder of North Koera, Kim Il-sung, was born on the same day as the Titanic sank.