If you try to divide 365 by 7, it does not come out evenly. 364, however, does divide evenly, meaning that if you divide 365 by 7, you get a remainder of one. This fact, when coupled with information on leap years (every 4 years), non-leap years (what should be leap years, but end in 10) and “re-instated” leap years (years that end in 10 that should be a leap year, but also are divisible by 200, such as 2000 was), one is able to use this information to fairly easily learn what day of the week any day in history falls on.

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