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The Doomsday Rule is an algorithm that makes computing the day of the week for a given date possible (and easy). Every year has an "anchor" day of the week, or "doomsday" according to Conway, where many easy-to-remember dates fall. Memorize those dates, learn to compute the anchor, and you, too, can count the days until doomsday like a pro. Numberphile recently put out a video about this, and Mike Boyd just put out a great video showcasing how easy it is to learn. Also see It's Okay To Be Smart.