When Is (American) Thanksgiving Most Likely To Happen?

Today (the 26th of November) is the Thanksgiving holiday in the United States. The holiday’s set, by law since 1941, to the fourth Thursday in November. (Before then it was customarily the last Thursday in November, but set by Presidential declaration. After Franklin Delano Roosevelt set the holiday to the third Thursday in November, to extend the 1939 and 1940 Christmas-shopping seasons — a decision Republican Alf Landon characterized as Hitlerian — the fourth Thursday was encoded in law.)

Any know-it-all will tell you, though, how the 13th of the month is very slightly more likely to be a Friday than any other day of the week. This is because the Gregorian calendar has that peculiar century-year leap day rule. It throws off the regular progression of the dates through the week. It takes 400 years for the calendar to start repeating itself. How does this affect the fourth Thursday of November? (A month which, this year, did have a Friday the 13th.)

It turns out, it changes things in subtle ways. Thanksgiving, by the current rule, can be any date between the 22nd and 28th; it’s most likely to be any of the 22nd, 24th, or 26th. (This implies that the 13th of November is equally likely to be a Friday, Wednesday, or Monday, a result that surprises me too.) So here’s how often which date is Thanksgiving. This if we pretend the current United States definition of Thanksgiving will be in force for 400 years unchanged:

November	Will Be Thanksgiving
22	58
23	56
24	58
25	56
26	58
27	57
28	57
	times in 400 years

I hope this helps your planning, somehow.

When Is Thanksgiving Most Likely To Happen?

I thought I had written this up. Which is good because I didn’t want to spend the energy redoing these calculations.

The date of Thanksgiving, as observed in the United States, is that it’s the fourth Thursday of November. So it might happen anytime from the 22nd through the 28th. But because of the quirks of the Gregorian calendar, it can happen that a particular date, like the 23rd of November, is more or less likely to be a Thursday than some other day of the week.

So here’s the results of what days are most and least likely to be Thanksgiving. It turns out the 23rd, this year’s candidate, is tied for the rarest of Thanksgiving days. It’s not that rare, in comparison. It happens only two fewer times every 400 years than do Thanksgivings on the 22nd of November, the (tied) most common day.

When Is Thanksgiving Most Likely To Happen?

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.

A Thanksgiving Thought Fresh From The Shower

It’s well-known, at least in calendar-appreciation circles, that the 13th of a month is more likely to be Friday than any other day of the week. That’s on the Gregorian calendar, which has some funny rules about whether a century year — 1900, 2000, 2100 — will be a leap year. Three of them aren’t in every four centuries. The result is the pattern of dates on the calendar is locked into this 400-year cycle, instead of the 28-year cycle you might imagine. And this makes some days of the week more likely for some dates than they otherwise might be.

This got me wondering. Does the 13th being slightly more likely imply that the United States Thanksgiving is more likely to be on the 26th of the month? The current rule is that Thanksgiving is the fourth Thursday of November. We’ll pretend that’s an unalterable fact of nature for the sake of having a problem we can solve. So if the 13th is more likely to be a Friday than any other day of the week, isn’t the 26th more likely to be a Thursday than any other day of the week?

And that’s so, but I’m not quite certain yet. What’s got me pondering this in the shower is that the 13th is more likely a Friday for an arbitrary month. That is, if I think of a month and don’t tell you anything about what it is, all we can say is it chance of the 13th being a Friday is such-and-such. But if I pick a particular month — say, November 2017 — things are different. The chance the 13th of November, 2017 is a Friday is zero. So the chance the 26th of December, 2017 is a Thursday is zero. Our calendar system sets rules. We’ll pretend that’s an unalterable fact of nature for the sake of having a problem we can solve, too.

So: does knowing that I am thinking of November, rather than a completely unknown month, change the probabilities? And I don’t know. My gut says “it’s plausible the dates of Novembers are different from the dates of arbitrary months”. I don’t know a way to argue this purely logically, though. It might have to be tested by going through 400 years of calendars and counting when the fourth Thursdays are. (The problem isn’t so tedious as that. There’s formulas computers are good at which can do this pretty well.)

But I would like to know if it can be argued there’s a difference, or that there isn’t.