Doomsday Algorithm was invented by Damian Conway. It's a neat algorithm that allows me to say that the 13th of September was on a Tuesday this year. Or that a) 27th of September is on a Monday and b) Christmas Day is on a Saturday.
The mental math isn't that hard to master, but it's quite rewarding to be able to look at your wife and say "Honey our anniversary is on a Tuesday this year, what would you like to do?" anytime you want without having to memorize the exact day.