the Jester
Legend
I missed this last year, but I think it's a pretty cool idea.I'm on board. It makes a worthier hobby-wide celebration IMO than May the Fourth (Be With You), Int'l Talk Like a Pirate Day, or even Gary Gygax Day.
That said, it's extremely niche- most gamers probably don't have a clue about open licenses or SRDs or the like. I don't think it's got nearly as widespread a fan base as the others you mentioned.
I love the idea of releasing free open content to celebrate; might have to crank something out.