Mentally compute the day of the week (the perpetual calendar made easy)

By Alain Bellon • October 29, 2008

October 29, 2008 is… wednesday!

Being able to know the day of the week for any particular date is a handy ability. In our times, a cellphone or computer can answer the question in seconds, but it is more handy to just know.

Above all, being able to do it is impressive, which is of course the main reason for learning how to do it.

All methods involve the simple notion of counting by sevens from a date where the day of the week is known. The issue is complicated because calendar dates are not just consecutive numbers but a series of month days which alternate between 30 and 31, except for February. Then you have the added complication of leap years.

To make the method practical people have devised lists of numbers to assign to years and lists of numbers to assign to months. With a few rules and 4 mental computations the day of the week can be determined.

Over the years I have tried several variations on the methods, and found them all to be too cumbersome to be practical. So I created my own method.

The Bellon method

in 2 steps

If you can remember you will be able to tell the day of the week for any date in just seconds. The number sequence 033614625035 represents each month of the year (a month per digit).

First you take your date, add the month digit, and add 2 (for the year 2008, explained below).
Now all you do is obtain the remainder of your total divided by 7.

Once I got the month digits memorized, it takes me 2-3 seconds to come up with the answer. It’s really easy.

Examples:

What day of the week was the 12 of November of 2008?

Take 12 and add 3 (november digit) add 2 (year digit) = 12 + 3 + 2 = 17.
Now divide 17 by 7, and you have a remainder of 3. The 2nd day of the week is Wednesday.

What day of the week was the 25 of May 2008?