Significant Reference Date

As an Easter Egg, writers will sometimes use a Real Life calendar date as an In-Universe reference point.

This could be a reference to a TV episode's original air date or a movie's original release date. Or perhaps a character's birthday, anniversary, etc. matches that of the actor who plays him.

Note: even when this is not Canon, Fanon is likely to assume this is true for works set in the period they were filmed in and characters in those works. Fanfic for that work will be affected accordingly.

Anime and Manga

 * Inverted by the Japanese DVD release of The Disappearance of Haruhi Suzumiya, which was released on December 18, 2010. (December 18 is the date in-story when weird stuff starts happening). Taken even further by Fanon, which believes that the exact date of the "three-years-ago" Tanabata was July 7, 2007, making the exact in-story date of Disappearance December 18, 2010.
 * Two different Gundam characters (Wings Relena Peacecraft and 00s Setsuna F. Seiei) were born on April 7th, the date the original Mobile Suit Gundam debuted in 1979; for Relena it's doubly significant, since her series premiered on that same date in 1995.
 * Most of the characters in Neon Genesis Evangelion share birthdays with their voice actors.

Film

 * The V for Vendetta film adaptation tried to invert this by setting a release date of November 5th, 2005, but ended up being delayed to March 2006.
 * Caveman, the 1981 comedy starring Ringo Starr and financed by George Harrison, was produced in memory of the recently slain John Lennon. The opening titles note the film takes place "One Zillion BC, October 9th" - Lennon's birthday (the date, not the year).
 * During the scene in Aliens where Ripley confronts Burke, she mentions that his order to the colonists was dated "6/12/79", which was Alien's release date.

Literature

 * Several characters in Harry Potter have Significant Birthdates. Additionally, several of the events in the books fall on special dates. Voldemort's first meeting with baby Harry happens on Halloween. In the final book, the Battle of Hogwarts happens on May 1st, Walpurgis Night, the feastday of St. Walpurgis, known also as "the witches' sabbath". As the sun rises on the end of the battle, it is the end of the witching time.

Live Action TV

 * In Community's Christmas Episode "Abed's Uncontrollable Christmas", Troy says "Today's December 9th"... the same date as the episode's original 2010 air date.
 * Also done in the previous season's Christmas episode. Shirley chastises Jeff for disrespecting the holiest day of the year, to which Jeff responds that it's December 10.
 * In the Remington Steele episode "Elegy in Steele", Steele's arch-enemy, Major Descoine, returns and promises that Steele and Laura Holt would be dead by noon that day. As part of his terror campaign, he sees to it that they stumble across headstones with their names and that day's date on them. That date was the original broadcast date.
 * In the final episode of Seinfeld, the events brought up as evidence against the gang are referenced as happening on the original air dates.
 * In the episode of The Big Bang Theory entitled The Excelsior Acquisition, Sheldon finds a traffic summons conserning an traffic incident that occured on 11/16, the airdate of The Adhesive Duck Deficiency in which the violation happenend.
 * The episodes of The Middle and Modern Family - both titled Leap Day - originally aired Feb 29, 2012.
 * In the I Love Lucy episode "Lucy Gets A Paris Gown", Lucy wants to get a new designer dress while in France. Ricky reads a letter Lucy wrote in Hollywood, promising Ricky if he'd buy her a Hollywood designer dress, she would never ask for another designer dress. The note is dated 2/28/55 -- The broadcast date of the episode "The Fashion Show" in which Lucy gets said dress. Extra coolness points awarded when you realize the writers had no idea what a rerun was at this point...let alone that anyone in the future would create episode guides, listing airdates.
 * In the fifth series of the new Doctor Who, Amy's wedding is revealed in "Flesh and Stone" to happen on the broadcast date of "The Big Bang".
 * "The Waters of Mars" tried to do this: the story was set on November 21, 2059, but the episode aired on November 15 due to a scheduling conflict.
 * In an episode of Maverick, Bret Maverick's birthday is given as the same as that of James Garner.
 * At the end of M*A*S*H episode "Welcome to Korea" (B.J.'s first episode), the PA announces the date that Col. Sherman Potter would arrive to take command of the 4077th. That date was the date of the broadcast of the next episode, in which Potter is introduced.
 * A screen in the Star Trek: Enterprise episode "In a Mirror, Darkly Part 2" gives James T. Kirk's birthday as March 22, which is William Shatner's birthday.
 * In Power Rangers RPM, Summer asks if Project Go-Onger was built in 1975, which happens to be the debut year of the Super Sentai franchise from which Power Rangers pulls its fight footage and costumes from.
 * The TV Land special Brady -- An American Chronicle note Brady events as taking place on the original broadcast dates.
 * Pretty Guardian Sailor Moon showed enough calendars on-screen -- particularly during the episode broadcast during New Year's week -- to make it clear that the show was progressing in real time from 2002 to 2003.

Western Animation

 * In the pilot episode of The Looney Tunes Show, Bugs Bunny and Daffy Duck's birthdays are stated as being July 27th and April 17th, the dates that their first theatrical shorts were first released.

Video Games

 * The HD version of Sonic Unleashed has domesticated Egg Fighters in the Eggmanland entrance called "EF-MD1991", "EF-DC1998" and "EF-XB2006".