It was an old piece of information given in Julie Schwartz's "Answer Man" letter columns that Superman's birthday was on February 29th - meaning that Superman only had a birthday once every four years! Apparently, this is Superman's equivalent birthday according to the earth calendar; Krypton's calendar was slightly different, with eighteen Kryptonian years for every twenty-five years on Earth. Presumably, Superman also has a Kryptonian date for his birth that he celebrates, just as Jewish children have a birthday in the Hebrew calendar (where one's bar mitzvah is celebrated) and a birthday in the Western calendar.
This date of February 29th was not always his birthday, however. During the Golden Age, Superman's birth on Kryptonopolis was in October, according to ACTION COMICS #149 (1950).
What is even more interesting is the idea that Superman and Clark Kent have different birthdays!
SUPERMAN #249 (1972) by Cary Bates (which is incidentally also the first appearance of Terra-Man), has Superman succumbing to emotional problems that Kryptonians get on their sixth birthday. This date cannot be correct, however, as that would mean that Superman is then celebrating his 24th Birthday instead of the traditional 29 years of age! Assuming Bates made an error here, if he was 29 through most of his hero years, come this 1972 issue, it must mean he is 32, or having celebrated eight birthdays.
Superman gets another birthday in Alan Moore's SUPERMAN ANNUAL #11 (1985) which means that around the time of the Reboot, Superman was around 36 earth years old, and this was his ninth birthday. This annual also brings to mind one of the funniest questions of Earth-1: what DO you buy for Superman, anyway? Not since Bouncing Boy's quest to outdo the other male Legionnaires and search for a present to win the hand of Dream Girl, was there a more brain-rattling gift conudrum!
This slightly revised timeline fits in with the dates we have for Supergirl. Shortly after Kara Zor-El's appearance on earth, she has a birthday party and is stated to be "sixteen." If five years passed from the 1960s to the 1970s, that would mean that Supergirl is 21 Earth-years old in Paul Kupperberg's NEW ADVENTURES OF SUPERGIRL and her death in CRISIS, which makes her fashoin model and actress career brief but entirely plausible.
Interestingly, Clark Kent has a different birthday than Superman: According to SUPERMAN #263 (1973), Kent's birthday is June 18th. This was stated as being the date that Superman arrived on Earth, his "Earthday." In ACTION COMICS #241 (1958) it is learned that Superman celebrates the date of his coming to earth as well as his "true" birthday (because apparently every four years doesn't quite cut it). The date at the beginning of #241 is given as June 10, 1958, and that Superman's "Earthday" must have several days leeway from here. This is consistent with the June 18th date that was later given.
(Intriguingly, this is also consistent with the anniversary of Superman in the "real" world, where the first issue of ACTION COMICS appeared on the stands in June 1938!)
In Superboy's sixteenth (or really, fourth) birthday, according to Cary Bates's NEW ADVENTURES OF SUPERBOY #1 (1980) it was explained that the Kents placed an extra candle on Superboy's birthday cake in honor of a birthday where he almost was aged by aliens, a reminder of the time he lost.
The only place that we get a YEAR of birth for Superman is in the first "Superman of 2965" story, where it was stated that the original Superman was born in 1920, a suspect date considering that would make Superman's considerably older in his 1960s-1970s appearances. This date also lends significant credence to the theory that the "Superman of 2965" future is the future of Earth-2, not Earth-1, as Earth-1 has the Legion and Earth-2 does not, and also no person seen in the 2965 future (Joker, Batman) does not have equivalents on Earth-1. Also, it should be noted that Earth-2 Superman got married and presumably may have children, whereas Earth-1 Superman never did.