While it's well known Kermit the Frog was born in a swamp, the precise date of that birth is uncertain, with different months and days specified over the years. It's unanimously agreed upon that 1955 is the birth year of Kermit, marking not only his creation as a Muppet but the fictional character's literal "birth."
The earliest reference to Kermit's birthday came in an appearance on Dinah!, hosted by Dinah Shore on April 13, 1977. In that episode, Kermit celebrated his 21st birthday. As Karen Falk noted in an "Ask Henson" column circa 1999, "Kermit first appeared on television in 1955 (so he would have been 22 in 1977, but many show business people fudge their ages a bit!)." Falk also added that "the official year is 1955, but there is no official date. Besides - do you count when the egg was laid or when the tadpole lost his tail?"
In episode 406 of The Muppet Show, which was taped in May 1979 and aired that September, Miss Piggy arranges a birthday tribute show for Kermit. However, the frog claims that his birthday is "about four months" away.
In 2005, with the 50th anniversary of Kermit's "birth," the issue of assigning a specific date resurfaced. A National Public Radio Morning Edition report, broadcast on May 9, claimed that Kermit was born on that day; the actual broadcast focused on Kermit's "birth" as a puppet creation as opposed to an actual celebrity, and chose May 9 since that was the date Sam and Friends debuted. The date was repeated on Wikipedia, in personal blogs, and other materials in 2005. However, most print news periodicals referred to "Kermit's birthday" only in terms of years. In contrast, MSNBC, in an August 21, 2005 article, claimed that Kermit would turn 50 "next month," possibly alluding to Jim Henson's own birthdate of September 24th.
In 2006, the May 9th date cropped up again, in a Ford press release promoting the debut of Kermit's Ford commercial during the Super Bowl on February 6th, 2006. The press release, which treated Kermit not as a puppet but as genuine "celebrated actor, author, singer and journalist," included a biography of Kermit, which gave his birth date as May 9, 1955. However, when Kermit guest starred on Martha on October 5, 2006, the guest profile on the show's website insisted that Kermit was "'born' on May 9, 1955, in the mind of puppeteer Jim Henson," as opposed to a literal birth date.
The Muppets 2008 Day-at-a-Time Calendar also states Kermit's birthday as May 9th; the calendar page for that day pictures Kermit holding a present, with the words "Happy 53rd Birthday, Kermit!"
In his book Sesame Street Unpaved, David Borgenicht gave Kermit's birthday as February 29th, Leap Year Day. Since this is the only mention of this date, this could be interpreted as a joke about frogs leaping, and not as Kermit's official birthday.
In episode 106 of The Muppet Show, when Kermit and Jim Nabors discuss astrological signs during the talk spot, Kermit claims to be a Taurus, which would make his birthday between April 20 and May 20. While that's obviously a coincidence, it does give credence to May 9th as Kermit's birthday.