The Gregorian Calendar Is Mathematically Biased Toward Certain Weekdays

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The Gregorian calendar operates on a precise 400-year cycle of exactly 146,097 days, which does not divide evenly by seven, meaning dates cannot fall equally on all weekdays. As a result, some dates land on certain days of the week more frequently than others over any 400-year span. For example, January 1st falls on Sunday, Tuesday, or Friday most often, while Monday and Saturday are the least common. The bias varies by date depending on its position relative to February and the leap-year rules, which drop three century leap days every 400 years. This also means wall calendars repeat on predictable cycles of 6, 11, or 28 years, with 2025's calendar set to recur in 2031.

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