## 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.

## My Mathematics Blog, As March 2015 Would Have It

And now for my monthly review of publication statistics. This is a good month to do it with, since it was a record month: I had 1,022 pages viewed around these parts, the first time (according to WordPress) that I’ve had more than a thousand in a month. In January I’d had 944, and in February a mere 859, which I was willing to blame on the shortness of that month. March’s is a clean record, though, more views per day than either of those months.

The total number of visitors was up, too, to 468. That’s compared to 438 in January and 407 in short February, although it happens it’s not a record; that’s still held by January 2013 and its 473 visitors. The number of views per visitor keeps holding about steady: from 2.16 in January to 2.11 in February to 2.18 in March. It appears that I’m getting a little better at finding people who like to read what I like to write, but haven’t caught that thrilling transition from linear to exponential growth.

The new WordPress statistics tell me I had a record 265 likes in March, up from January’s 196 and February’s 179. The number of comments rose from January’s 51 and February’s 56 to a full 93 for March. I take all this as supporting evidence that I’m better at reaching people lately. (Although I do wonder if it counts backlinks from one of my articles to another as a comment.)

The mathematics blog starts the month at 22,837 total views, and with 454 WordPress followers.

The most popular articles in March, though, were the set you might have guessed without actually reading things around here:

I admit I thought the “how interesting is a basketball tournament?” thing would be more popular, but it’s hampered by having started out in the middle of the month. I might want to start looking at the most popular articles of the past 30 days in the middle of the month too.

The countries sending me the greatest number of readers were the usual set: the United States at 658 in first place, and Canada in second at 66. The United Kingdom was a strong third at 57, and Austria in fourth place at 30.

Sending me a single reader each were Belgium, Ecuador, Israel, Japan, Lebanon, Mexico, Nepal, Norway, Portugal, Romania, Samoa, Saudi Arabia, Slovakia, Thailand, the United Arab Emirates, Uruguay, and Venezuela. The repeats from February were Japan, Mexico, Romania, and Venezuela. Japan is on a three-month streak, while Mexico has sent me a solitary reader four months in a row. India’s declined slightly in reading me, from 6 to 5. Ah well.

Among the interesting search terms were:

• right trapezoid 5 (I loved this anime as a kid)
• a short comic strip on reminding people on how to order decimals correctly (I hope they found what they were looking for)
• are there other ways to draw a trapezoid (try with food dye on the back of your pet rabbit!)
• motto of ideal gas (veni vidi v = nRT/P ?)
• rectangular states (the majority of United States states are pretty rectangular, when you get down to it)
• what is the definition of rerun (I don’t think this has come up before)
• what are the chances of consecutive friday the 13th’s in a year (I make it out at 3/28, or a touch under 11 percent; anyone have another opinion?)

Well, with luck, I should have a fresh comic strips post soon and some more writing in the curious mix between information theory and college basketball.

## Something I Didn’t Know About Trapezoids

I have a little iPad app for keeping track of how this blog is doing, and I’m even able to use it to compose new entries and make comments. (The entry about the lottery was one of them.) Mostly it provides a way for me to watch the count of unique visits per day, so I can grow neurotic wondering why it’s not higher. But it also provides supplementary data, such as, what search queries have brought people to the site. The “Trapezoid Week” flurry of posts has proved to be very good at bringing in search referrals, with topics like “picture of a trapezoid” or “how do I draw a trapezoid” or “similar triangles trapezoid” bringing literally several people right to me.