Today Is Superman's Birthday
The Man of Steel's birthday is traditionally celebrated on Leap Years only.
Today is February 29, which makes it the day Superman's birthday is recognized in popular culture. For years, both Superman's age and birthdate have been variable in the comics, but the most consistently-used birthday for Superman is February 29. The in-real-life joke is that since Superman never seems to age, it makes sense that he has a birthday that only comes around once every four years. In the comics, there was at one point a similar logic that held Krypton's year was significantly longer than Earth's. Of course, Superman's birthdate is more or less an illusion anyway, since "Superman" is a creation of Clark Kent. Even if you want to distinguish between Kal-El and Clark, we don't know what Kal-El's birthday would have been, since Krypton didn't seem to have Leap Years.
Clark Kent, meanwhile, will celebrate his birthday on June 18, the date the Kents first found the rocket in their Kansas corn field. June 18 is also the birthday of actor Bud Collyer, who was the first actor to play Superman (in The Adventures of Superman radio show).
In 1985, one of the best-loved Superman stories of all time was published: "For the Man Who Has Everything," published in Superman Annual #11 by the Watchmen creative team of Alan Moore and Dave Gibbons, took place on Superman's birthday. That story was adapted into an episode of Justice League Unlimited, and was the basis for an episode of Supergirl.
In the story, Mongul traps Superman by "gifting" him a Black Mercy flower, an alien symbiotic plant that connects itself to a host's mind and giving the host visions of their "perfect," ideal world, lulling them to sleep. In the case of Superman, he saw a version of his life where Krypton had never been destroyed, and then was furious when he discovered that he had to lose it all over again. You can buy it digitally here.
February 29 is also the birthday of Michael Bailey, Superman super-fan and co-host of From Crisis to Crisis: A Superman Podcast, which goes issue-by-issue covering the Superman comics set between John Byrne's The Man of Steel #1, published just after Crisis on Infinite Earths, and the start of Infinite Crisis, which marked huge changes for Superman as a character and restored the multiverse, effectively ending the 20-year-old "one-universe" status quo. The show is pretty great, and Bailey does quite a lot of other podcasting about comics, too. Just don't ask him to take his glasses off.