So apparently this is a thing, and has been since 1995, and this is the first time I've heard of it. I have no excuse, and neither do you. But now that we do know, let us put aside our indifference and celebrate with a few well-chosen links--
There are other ways to celebrate of course--watching superhero TV or movies, superhero gaming, superhero cosplay--to name but a few. Enjoy the day, and maybe we can plan something for next year.