Starting in the Silver Age, DC used February 29th as the birthday for the Last Son of Krypton! The reason behind this was a bit of a tongue-in-cheek poke at Superman having been around for multiple decades but seemingly never aging, so the choice was made to give him a birthday only once every 4 years!
Of course multiple days have been used to celebrate Superman's birthday over the years. There's June 18th, the date celebrated by Clark Kent as his birthday, this is typically the day used for the day the Kent's discovered his rocket. Of course this was not the day used Post-Crisis, as Superman's birthing matrix arrived right before a giant winter blizzard
1950's Action Comics #149 lists October as the month of Superman's birthday. Let's just explain these discrepancies as representing different Me of Steel throughout the DC Multiverse.
Happy Birthday Superman & many happy returns! May this Monday find you in good health Man of Steel!
Enjoy some of these classic Superman Birthday & Anniversary images from over the years! And if you feel so inclined, go celebrate by picking up the latest issue of Dan Jurgens and Lee Week's fantastic Superman: Lois & Clark series, as well as the debut first issue of Neal Adam's Supermenboth of which went on sale last Wednesday.
And I'd also like to throw-out a big Happy Birthday to friend of the show, and fellow Superman enthusiast, Michael Bailey of the From Crisis to Crisis and Radio KAL Live Superman themed podcasts. Happy Birthday Mike, have a SUPER Day!