If you don't have any fun plans yet, you can find fun events in your area here.
I'm having a belated Darwin Day celebration. The Seattle Atheists are having their party on Sunday the 13th. They'll be at the Juanita Community Club from 1 to 4:30 pm with games, prizes, food (including birthday cake!). The event is family friendly, so you can bring your kids too!
I hear there will be a game of "Phylum Feud," which I plan to own. Or more likely, be an embarrassed biologist when I fail miserably.
Though if you're on the opposite side of the country, you should check out the event Harvard is having. They're hosting the exclusive New England screening of Matthew Chapman's (Darwin's great great grandson) new thriller The Ledge. It's about atheism! And Christianity! And a "lethal battle of wills"! You can get more information about the free event here.