Actually, it's not quite true that no one celebrates it; when I reported on NCBD last year, a couple of impromptu events had sprung up, and the Patch local-news sites were having some fun with it. Things seem to be quieter this year, if only because Patch has shut down a lot of its sites, and therefore the local activities may be going unreported.
Without significant retailer, publisher or grass-roots support, National Comic Book Day exists solely as an excuse for geeky writers to pitch comics stories to their editors, such as this top five comics series article at Syracuse University's Daily Orange, or for a nerdy weather reporter to throw a comics reference into the daily forecast.
The most comprehensive description I saw was actually on a Russian-language site. I don't read Russian, but as it's always fun to see your culture through someone else's eyes, here's the Google translation:
25 September commemorates National day of comics (National Comic Book Day). On this day they are going to the clubs, specialty stores, at exhibitions and fairs. Many have become millionaires, but hardly dare to sell their collections.
Comics are the ancestors of Europeans. And the first American comic strip, "The cubs and tiger" by the artist Richard Outkaulta was released in 1892.
Despite this, I still think Alex Zalben's 2012 article 10 Great Ways to Celebrate National Comic Book Day is the definitive article on the topic.
Incidentally, today is also Family Day, Math Storytelling Day and National One-Hit Wonder Day, which suggests a pretty awesome combined celebration.
How are you observing National Comic Book Day? Let us know in the comments!