So my question from last Thursday nagged at my mind. And I learned that Octave (a Matlab clone that’s rather cheaper) has a function that calculates the day of the week for any given day. And I spent longer than I would have expected fiddling with the formatting to get what I wanted to know.
It turns out there are some days in November more likely to be the fourth Thursday than others are. (This is the current standard for Thanksgiving Day in the United States.) And as I’d suspected without being able to prove, this doesn’t quite match the breakdown of which months are more likely to have Friday the 13ths. That is, it’s more likely that an arbitrarily selected month will start on Sunday than any other day of the week. It’s least likely that an arbitrarily selected month will start on a Saturday or Monday. The difference is extremely tiny; there are only four more Sunday-starting months than there are Monday-starting months over the course of 400 years.
But an arbitrary month is different from an arbitrary November. It turns out Novembers are most likely to start on a Sunday, Tuesday, or Thursday. And that makes the 26th, 24th, and 22nd the most likely days to be Thanksgiving. The 23rd and 25th are the least likely days to be Thanksgiving. Here’s the full roster, if I haven’t made any serious mistakes with it:
November | Will Be Thanksgiving |
---|---|
22 | 58 |
23 | 56 |
24 | 58 |
25 | 56 |
26 | 58 |
27 | 57 |
28 | 57 |
times in 400 years |
I don’t pretend there’s any significance to this. But it is another of those interesting quirks of probability. What you would say the probability is of a month starting on the 1st — equivalently, of having a Friday the 13th, or a Fourth Thursday of the Month that’s the 26th — depends on how much you know about the month. If you know only that it’s a month on the Gregorian calendar it’s one thing (specifically, it’s 688/4800, or about 0.14333). If you know only that it’s a November than it’s another (58/400, or 0.145). If you know only that it’s a month in 2016 then it’s another yet (1/12, or about 0.08333). If you know that it’s November 2016 then the probability is 0. Information does strange things to probability questions.
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