Today is the 256th day of the year. If you start the year with day 0, today is # 255 = FF hex = 11111111 binary = the highest unsigned integer you can represent in 8 bits = #8bitday.
I tweeted this last year (created a wiki page), and then discovered Programmer's Day on Wikipedia (I think someone on Twitter pointed it out). Apparently it's an official professional holiday in Russia!
While I certainly appreciate the recognition (being a programmer myself, and having thought in 8 bits for far too many years), I prefer a broader appeal - hence, "8 Bit Day" in recognition of 8 bit art & culture in general, which anyone can appreciate.
This year the most prominently public celebration of 8 bit culture is no doubt the movie Scott Pilgrim vs. The World (and the graphic novels) which was packed full of 8-bit (and some 16 bit) video game references.
If you're an 8-bit fan and a Halo fan, you might also enjoy the 8-bit Halo machinima videos on YouTube.
Finally, if you like 8-bit music, AKA "chiptunes", check out: