Leap Day

The twenty-ninth day of February (usually) comes only once every four years. As the only date in the calendar that doesn't occur in every year, it has some comedic potential, especially when it's someone's birthday: Any event happening on someone's "Xth birthday" won't actually happen until he's 4 times X years old. People whose birthday falls on Leap Year's Day are known as "leapers" (not like that) or "leaplings."

Though this can be a hassle if said character wants to celebrate his birthday every year. One way to do this is to celebrate it on the first of March, so technically he'd still 'age' normally.

Anime and Manga

 * The Ouran High School Host Club manga explains that senior student Honey looks younger than the freshman students since he was born on February 29.
 * In Karin, Kenta was born on 29 Feb 1988. Elda threatens to wipe his memory back to his fourth birthday, not realizing that she is doing so on his fourth birthday.
 * In Gundam ZZ, Haman formally declares the founding of the Neo Zeon movement on February 29, UC 0088. Fans have used this to try and determine at what point the calendar transitioned from AD to UC.
 * A gag in Otaku no Musume-san revolves around Kanau's birthday being on February 29.
 * Touya Kinomoto from Cardcaptor Sakura also has his birthday on February 29.

Comic Books

 * Superman's birthday is February 29. Yes, really. To be more exact, it was Feb. 29 on Earth on the day he was born on Krypton. He celebrates the day the Kents found him separately, as Clark Kent's birthday.
 * In The Golden Age of Comic Books, Feb. 29 was also Captain Marvel's birthday. (Hey, maybe he and Superman were separated at birth!)
 * In a manner of speaking... Golden Age Marvel Family writer Otto Binder eventually went to work for DC on the Superman titles in the Silver Age, coming up with many of the familiar Superman mythos elements (Krypto the Superdog, the Phantom Zone, Supergirl, etc.). This might explain some of the tone/elements of Silver Age Superman stories...
 * It used to be that Leap Day was the day on which women could propose to men, instead of the other way around as was the informal custom of the time. Carol Ferris tried to take advantage of this in an issue of Green Lantern.

Film

 * Irish folklore has this as the day Saint Bridget struck a deal with Saint Patrick to allow women to propose to men. This is the premise of the 2010 Amy Adams film Leap Year.

Literature

 * In Whit by Iain Banks, the Luskentyrian cult consider those born on the 29th of February to be sacred, so they have an orgy nine months before that date.

Live Action TV

 * On Wings, Roy was born on Feb. 29 and held a birthday party for himself every four years, saying he was 10 when he was 40 or whatever the precise dates were. While Roy acknowledged his real age, the tradition was that everyone would throw him a kids' party based on his Leap Year age.
 * The Frasier episode "Look Before You Leap" takes place on a Leap Day. Frasier encourages everyone to "take a leap" and try something new. One of Martin's friends was also born on Leap Day.
 * Sue Heck is a Leap baby who dreamt of actually celebrating her 15th birthday on Feb 29, 2012.
 * Cameron on Modern Family is also a Leap Day baby. Phil also takes Feb. 29 off to enjoy this "bonus day".
 * Jerry on Parks and Recreation was born on this day.
 * Leap Day William emerges from the Marianas Trench dressed in blue and yellow on February 29. There is even a movie about him starring Jim Carrey.

Tabletop Games

 * In the Forgotten Realms setting of Dungeons & Dragons, there is a day called Shieldmeet, which occurs only once every four years on the day after Midsummer, making it an in-universe leap day.

Theater
"Through some singular coincidence--I shouldn't be surprised if it were owing to the agency of an ill-natured fairy-- You are the victim of this clumsy arrangement, having been born in leap-year, on the twenty-ninth of February, And so, by a simple arithmetical process, you'll easily discover, That though you've lived 21 years, yet, if we go by birthdays, you're only 5 and a little bit over!"
 * In The Pirates of Penzance, the Pirate King and Ruth inform Frederick that he is bound to serve them, not until his twenty-first year, but until his twenty-first birthday.

Video Games

 * In Temple of Elemental Evil Bertram winds up enslaved until his "twenty first birthday" (i.e. 84 years) due to his being born on this date, most likely a Shout-Out to The Pirates of Penzance.

Web Comics

 * In And Shine Heaven Now, this is how Annie Warbucks explains how she looked no older than 30 (in 1998) and still could be alive during World War II: because she was born on this day, she ages at the quarter of a rate of a normal human. Of course, she could just be pulling Seras's leg. (That particular comic was posted on April Fool's Day, after all.)
 * Howard Tayler of Schlock Mercenary is among those born on February 29.
 * Michelle from the webcomic Scrambled Eggs was born on February 29. The birthday parties this Dumb Blonde holds every four years have become Cool and Unusual Punishments for the other characters.
 * In one Nedroid comic, Leap Day goes horribly wrong.

Web Original

 * "It was April the 41st, being a quadruple leap year..."

Real Life

 * As noted, Irish folklore marks February 29 as the day women are allowed to propose to men, explain as a deal Saint Bridget struck with Saint Patrick.
 * As a curiosity, when Gaius Julius Caesar had the calendar reformed, the year 46 BC was made 445 days long to get up to date, making it the longest year in history.