At the start of this year, I was reminded of the Finger Calendar method my mom taught me when I was growing up. According to her method, you can figure out the day of the week for any date in history. You just need to calibrate your fingers for the given year.

For example, my birthday is October 19; on what day of the week will that fall in 2011? I can figure that out by tapping on the knuckles of my left-hand fingers in a particular pattern. And the answer is . . . Wednesday! (Not the optimal day of the week to have a birthday, but alas.)

I hadn’t used this system in many years and the details were fuzzy, so I began a quest to figure it out again. I turned to trusty Google but, surprisingly, didn’t find the algorithm. I found several websites that will figure out a date *for* you; and this website even has several different manual methods, but they’re quite a bit more complicated than the one I vaguely recall. So, I was going to have to relearn the Finger Calendar on my own.

I recalled that it had something to do with assigning knuckles to the months in a pattern, and then assigning those knuckles to the days of the week in a different pattern. I knew there must be seven knuckles involved, correlating to the seven days of the week. I recalled that it started at the top right knuckle, and months were counted 1–7 as shown above, and days of the week were counted in the A–G pattern. But I needed to figure out the pattern in which the months were arranged, and which day of the week started on A.

I looked at a calendar and wrote down the day of the week for the first day of each month this year.

January 1 = Saturday / February 1 = Tuesday / March 1 = Tuesday

April 1 = Friday / May 1 = Sunday / June 1 = Wednesday

July 1 = Friday / August 1 = Monday / September 1 = Thursday

October 1 = Saturday / November 1 = Tuesday / December 1 = Thursday

Since January 1 is a Saturday, I knew A was Saturday, and counted from there to arrange the days of the week like this:

Next, I needed to arrange the months, so I placed them on the knuckle that correlated to each month’s first day. That gave me this pattern:

Almost there!

I vaguely recalled this pattern, tapping and reciting: 1-“January,” 2-“February,” 2 (again)-“March,” 3-“April,” 4 (back to the right)-“May,” 5-“June,” 3 (up to the top again)-“July,” 6 (back the the right again)-“August,” 7-“September,” 1 (upper right now)-“October,” 2- “November,” 7 (the weird one, drop down)-“December.”

Okay, now to figure out a given date. Let’s take June 21, the summer solstice. First, tap and recite the months, ending at June. That’s knuckle 5 (or E). That knuckle represents the first day of the month. Now, tap in the days-of-week pattern until you get to the 21st. So count and tap: E = 1st, F = 2nd, G = 3rd, A = 4th, etc., down the knuckles until you land on 21, knuckle D, which is Tuesday.

Let me go look at a calendar and see if I’m right . . . Yes! I’ve calibrated the finger calendar!

So that’s how I used math reasoning during my winter break to recreate an algorithm.

I’ve always believed that the purpose of math education is not to teach you facts and algorithms that you’ll recall and use for decades; rather, its purpose is to introduce you to what is possible with mathematics and give you the skills to recreate algorithms or look them up later as needed. Using algorithms to solve “real world” problems—for instance, finding out when your birthday falls—is not only helpful; it helps explain math concepts in a way that’s relatable and, if you ask me, fun. Once again, mother knows best.

Hey Josephine, thanks for this little blog. I used to do this when I was young. I still have it and know how it works. If you want the instruction set, send me your email address and I will show you how on a Word Doc.

You also made a mistake, the inside of the hand should be facing to you as you use the thumb as a Pointer. 🙂

Hi there, I just came across your comment regarding finger calculators on Sine of the Times blog and I saw you offered the author the instructions in a Word Doc and I’m wondering if you’d be so kind as to extend those instructions to me? Thank you very much!!

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