A Neat Calendar Trick!

Ever wonder on what day of the week your parents were born? There is an easy way to figure out what day of the week any date occurred on in past, and even future years.

I never really thought about how the calendar looked in past years, until I was at my grandmother’s house one day years ago. She was using a special made calendar, that was dated from back in the late 1930s, early 1940s. Something else I noticed about the calendars was that though the years were incorrect (for the current year), the placement of the dates for any given month was correct. Let’s look at the month of June, in 2019.


June 2019

S M T W T F S

1

2 3 4 5 6 7 8

9 10 11 12 13 14 15

16 17 18 19 20 21 22

23 24 25 26 27 28 29

30

Below, is the calendar for June 1879, many years before all of us were born.


June 1879

S M T W T F S

1

2 3 4 5 6 7 8

9 10 11 12 13 14 15

16 17 18 19 20 21 22

23 24 25 26 27 28 29

30

Notice anything interesting? The dates align exactly. If you want verification, go to www.timeanddate.org and type in both years. Look at the month of June, and see if they match. One may wonder, how I figured this out? There’s a simple trick that allows a person to figure this out. There are a few things to keep in mind. Firstly, every full week in a month will have 7 days. Also, in every year that isn’t a leap year all dates will move exactly one place forward on the calendar. Christmas this year will fall on a Friday, and next year it falls on a Saturday. This pattern is true for every year. Leap years are the only other factor that can affect when a day falls. Every year that is divisible by 4 is a leap year, and there are 29 days in February that year. The extra day will cause all dates beginning March 1st through the rest of the year to move two places on the calendar. This year as I mentioned earlier, Christmas is on Friday. Last year it was on a Wednesday. The current year, 2020, is a leap year since it is divisible by 4, hence Christmas moved two places on the calendar since the current year is a leap year, and December falls after February.

What does this have to do with the calculation of determining dates in other years? There are 7 days in a week, so likewise every date in any given month can appear in any of the 7 positions on a calendar. I also mentioned that every 4 years is a leap year, that causes all dates after February to jump two days (January and February’s dates will jump an extra spot the following year). When you multiply the 7 days in a week by 4 to factor in leap years, you get 28. Twenty-eight here, is significant in two ways. Firstly, there are 28 different ways the calendar can appear before it repeats itself (some years will be duplicates, but leap years will always be different), and you will know what the calendar looked like in a previous or future year by adding or subtracting 28 from the year. This works for most years (the only years it doesn’t work for are years divisible by 400 which also do not contain a leap year).

Most people don’t know what past years or even months look like to make this useful, but there are a few other facts to keep in mind. In any non-leap year, the months of January and October look the same. The same is true for February, March, and November; April and July; and September and December. If you know one, you can figure out the other. May, June, and August do not have counterparts.

A person can also backtrack or move forward on any given year by adding or subtracting the dates on the Sunday column. Just remember to add or subtract the correct number between 28 and 31 as appropriate.

When people discover I have this ability, they like to ask me on what day of the week they were born. They give me the date and year, and I use the technique to calculate it, and then they check it on their cellular phone to see if I’m right. I am correct 99% of the time, and people are amazed. If I’m off, it’s by only a day. The way I do it quickly, is by picking a year that I know the calendar layout well, and then begin the repeated subtraction, until I get to a year close by the year required. Then I will work through the calendar out loud to get the exact year, and then the exact day of the week. This has impressed many people over the years.

The calendar trick is a neat trick that anyone can do. It isn’t practical for most people, since it requires knowledge of calendars from past years, or at least fast figuring of past years. Still there are little tricks, that can help a person traverse the calendar in their head since many months mirror other months. The take-away message is that, it’s a neat little trick that will wow people, and attract positive attention— something useful for people on the Autism Spectrum.

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