My middle school years were a time when I refused to go to school. Later, I lived the life of a hikikomori, withdrawing from all social interactions. A decade from then, in 2002, while I lay in bed because of a cold, I found myself thinking about a technique my father had taught me long before that involved using the fingers to figure out the day of the week of a given date. To use this skill, you first assign the seven days of the week to seven of the segments (the parts separated by joints) on three of the fingers (index finger, middle finger, and ring finger) on your left hand. You then use your left thumb as a “counter” by tapping the segments, which are used as aids to arrive at the correct day of the week. In Japan, this traditional skill called “finger counting” or “finger calendar” (*1) lets you calculate the day of any date in a year as long as you know which day of the week January 1st falls on that year. The caveat of this method is that you need to know which day January 1st is, by looking it up in a calendar etc. beforehand. I had learned on TV programs of the skill that people with savant syndrome(*2) possess to calculate the day of the week, so I wanted to devise a method to calculate the day of January 1st without having to look it up. Initially, I only had the vague goal of devising a method that would work for any date in the 20th century, but I greatly exceeded my initial expectations and found a completely new way to calculate the day of the week of a date in the 19th, 20th, 21st, or any other century! At first, I wasn’t so surprised; it was later that I realized how amazing this actually was, and I shivered in fear at my own discovery over and over again.
Using this new method(*3), I can calculate the day of any date after January 1st, 1601 on the Gregorian calendar (*4) within a set amount of time (approximately ten seconds on average). What I mean by “set amount of time” is that the maximum number of “counts” you need to do to get to an answer is about 30 (*5), so assuming you take about one second per count, a calculation only takes 30 seconds at most. Ten seconds is the estimated average time you need to calculate the day of the week (*6). When you start trying out this technique, in reality, it will take more time because it will take some time to make some of the decisions involved in the calculations as well as to do the calculations themselves. But in the same way that you need some extra time when you go somewhere for the first time but you get faster as you go there over and over again, you won't believe how fast your calculations will get with repeated practice. Instead of doing the calculations each time, you will see that you only need to draw on the memory of calculations you've done previously. What’s more, you'll eventually be able to do calculations in your head. I trained hard and mastered the new method. Now, I can calculate the day of the week for any date between January 1st, 1601 and December 31st, 11600 in 3–4 seconds (*7). I therefore believe that with practice, you, too, can acquire the genius of the "calendrical savants"! It's an amazing skill, so it's worth the try. You won't be disappointed.
(*1) There are a number of names that this traditional skill goes by, and there are many variations regarding the details. The historical background of the tradition is unknown.
(*2) The disorder became well known by the movie “Rain Man”. Savant syndrome patients typically suffer from autism and other disabilities but often develop genius-like abilities (probably resulting from certain disabilities). They are able to guess days of the week in the past correctly, just for one example.
(*3) This method is different from Tutorial.
(*4) While some countries started using it on October 15th, 1582, this system is currently the internationally accepted calendar. The earth’s orbit around the sun is approximated to a year, inserting a leap day in predetermined years to make adjustments. This system introduces an error of about 1 day every 3,300 years.
(*5) The maximum number of counts was in fact 32.
(*6) This is a rough estimate based on experiments and the average number of counts. The average number of counts for the period, January 1st, 1601–December 31st, 11600, is about 18.55.
(*7) This uses not the default method but a customized one, which I naturally developed after a lot of practice using the new method. See Archives.