When someone professes to have a photographic memory, the first thing many people ask them is to name the day of the week on which a random date landed. Often, they do it easily, and everyone around is sufficiently impressed with their memory. Unfortunately—or fortunately, for those without such a gift—naming the day of the week of a date doesn't require a photographic memory; just some quick mental arithmetic. Anyone can do it using the doomsday rule, a simple algorithm devised by mathematician John Horton Conway.
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