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--
Henry Cavill Celebrates National Superhero Day
Celebrating With Your Kids
Posts About National Superhero Day On Tumblr
Ben Affleck Recognizes Real-Life Heroes
National Superhero Day On Twitter
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.
Why is it damn near every day in the year has to be some "National XYZ day" or "International ABC" day??
If you're a comic book geek, then everyday is "National Superhero Day." All joking aside, I think it would be pretty cool to see more comic book-oriented programming at Gen Con that raises awareness about the actual holiday and what it hopes to accomplish.