What day of the Week was that?

Ever want to figure out what day of the week a certain day was, or what day of the week a date in the future will be?
Want to do it in your head?
You need the Doomsday Algorithm.
I wish that I could calculate it faster, but I think that I've got it down well enough to figure out for this year, maybe the next couple.
Cool way to impress your friends, or win a buck at the local watering hole.