MIKE KEELER: One More Round of Friday The 13th

(Oh, this is so lucky! It’s Friday the 13th, right smack-dab in the middle of August. Which means I can pull out an old post and take this week off. Whew!)

Attention paraskevidekatriaphobiacs: it’s worse than you think…

The Gregorian calendar was first rolled out in 1582, and was fully adopted by Europe in 1752. In order to make the thing align almost perfectly with the orbit of the earth around the sun, the Gregorian calendar uses 400-year cycles, with each cycle containing 97 leap days to keep things perfectly adjusted.

These 97 leap days fall in all years divisible by 4, BUT NOT in any years divisible by 100, UNLESS they are also divisible by 400. (For example, the year 1900 was not a leap year, but the year 2000 was. In fact, the year 2000 was the first year since the adoption of the Gregorian calendar that was subject to the “divisible also by 400” rule.)

Each 400-year Gregorian cycle therefore contains 146,097 days (365 X 400 + 97). Those 146,097 days are chunked into 20,871 weeks that each contain 7 days, while at the same time they are also chunked into 4800 months that contain either 28, 29, 30 or 31 days. One would think there would be a perfect statistical correspondence between days of the week, and dates of the month. But then one would be failing to account for the awesome power of PURE EVIL!

Looking at the dreaded 13th day of the month, it has a fondness for falling on a certain day of the week. In any 400-year cycle, the 13th will fall least often on Thursdays and Saturdays (684 times in the cycle). Monday and Tuesday get off easy (685 times). And Sunday and Wednesday are a little creeped out (687 times). Ah, but Friday – insert scary music here – Friday is friggin’ freaky. It gets to be the 13th more often than any other day of the week (688 times). That’s an increase over the average of, like, .0003%. Yikes!!

And so, it’s Friday the 13th. Maybe you’ve got the willies. Just a little freaked out. But try not to think about it. Relax. It’ll all be OK. It’s not like the entire cosmic deck is stacked against you.

Have a nice day.

Mike Keeler

Mike Keeler

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