In celebration, Bungie has several festivities planned, many of which are already in play. There are auctions, a Mobile App for iOS, Marathon for iPad, Bungie vs the World tournament where real steaks will be handed out (steak gift cards outside of the US), and blue flames for all.
Follow the instructions in the link above and you'll gain limited access to Blue Flames for your Spartan-III in Halo: Reach, as well as the Bungie nameplate until the end of Bungie Day.
You can check out the handy Red vs. Blue PSA here or below in case you have any more questions.
Thanks Bungie for being such a kick-ass developer that really knows how to keep your community going!