While everyone is talking about November 5th being significant because of Guy Fawkes, the folks over at /Film have pointed out a much more important thing that makes today special. It was 54 years ago today, on November 5th, 1955, that the eminent Doc Brown invented the flux capacitor which, of course, made time travel possible. (Provided, of course, you can find a way to generate 1.21 jigawatts of power.)
Obviously we can credit this invention with the greatness that is Back to the Future (and the not-so-greatness that is Back to the Future Part III) but also The Terminator, the wonderful Time Bandits and the fantastic 12 Monkeys. Those alone make this a great day indeed. However, with the good comes the bad, and because of this invention, we also got The Butterfly Effect and Time Cop, starring Jean-Claude Van Damme, so it's not all gravy.