It's February 29th, Leap Year Day, which can only mean one thing: It's Superman's birthday! Superman is, of course, 70 years old this year (ACTION #1 had an June, 1938 cover date, so that would have shipped in April, I think), but sometime in the 1970s it was decided that he was a Leap Year baby.
I asked Mark Waid if there was a citation in the comic books to this, and he replied: "I know it was in one of E. Nelson Bridwell's/Mort Weisinger's letter columns, a jokey answer about how Superman could have been an adult in 1938 and still look so young. Nelson 'cemented' the date in the 1976 Super DC Calendar from Warner Books."
I had a memory of reading a comic story, and Kurt Busiek says that SUPERMAN #249 deals with the birthday, but I don't have a copy of that to look up the details.
The Time Magazine article on Superman's 50th anniversary (3/14/88) also cites that as his birthday, but all of the links I found to that article 404ed.
Either way, mathematically, that kind of can't be right -- '38 wasn't a leap year, '36 and '40 were, but if we were to go with the earlier date, then Superman would be 18 today (kinda)
Either way, I love the idea of it, even if there's very little canon or facts to back it up, so Happy Birthday Superman!