Superman’s birthday is February 29th.
That’s right, the Man of Steel only has a birthday every four years –the key to his eternally youthful appearance!

art by Kurt Schaffenberger

February 29 is traditionally the day we celebrate Superman’s birthday. After playing around with a few different dates in the Golden Age, DC in the person of editor Julius Schwartz decided that Leap Day was the official day. An unusual day for an unusual fictional character. I remember the same day was used for Capt. Marvel back in the day as well, so maybe Schwartz was being referential as well as cute. Of course, there could be a sci-fi explanation as well, something about the length of a Kryptonian year being roughly equivalent to four Earth years? Schwartz mentioned the date in a few letters pages over the years but as far as I know, the actual date was only mentioned in one Pre-Crisis comic book story, the Alan Moore/Dave Gibbons “For the Man Who Has Everything” from Superman Annual #11, 1985.
Read a full discussion of the various conflicting dates of Superman’s birthday here.