Doomsday Algorithm

Doomsday Algorithm

This mindmap explains how the day of the week (Mon-Sun) can easily be determined for every day of the year from the top of your head! This is part of the Doomsday Algorithm (by Dr. John Horton Conway).
A good explanation (including how this works for other years [past and present] and even other centuries may I suggest: http://rudy.ca/doomsday.html. Google for more hits and wiki's.