Someone tells you their date of birth. Before they finish smiling, you tell them they were born on a Friday. That is a lovely party trick. It is also a good memory exercise, because it uses both calculation and memory.
In my book Memory Power, I explain the method I use myself. There are two parts. The first is a small formula. The second is memory, because you must remember a few codes.
The formula
You need three codes. The date code, the month code and the year code. Add them and divide by seven. Look at the remainder.
- Remainder 1 is Sunday.
- Remainder 2 is Monday.
- Remainder 3 is Tuesday.
- Remainder 4 is Wednesday.
- Remainder 5 is Thursday.
- Remainder 6 is Friday.
- Remainder 0 is Saturday.
If the total is less than seven, do not divide. The total itself is your remainder.
The date code is the easiest. It is the date itself. For the fifth of January, the date code is 5.
The month codes
These twelve numbers are the part you must memorise. January is 1. February is 4. March is 4. April is 0. May is 2. June is 5. July is 0. August is 3. September is 6. October is 1. November is 4. December is 6.
Say them in order a few times, as you would a phone number. Or turn them into pictures, as we do in the number chapters of the book.
The year code
The book gives a table of year codes for every year from 1901 to 2020, and rules for other centuries. For example, 1965 is 4. 1996 is 1. 1999 is 4. I will not print the full table here, because it is long and you will want it in front of you while you practise. It sits on pages 133 and 134 of the book.
There is one more rule. If the year is a leap year and the date falls in January or February, reduce one day from your answer. If you get Monday, the real day is Sunday.
Two worked examples
Take 5 January 1965. Date 5, month code for January 1, year code for 1965 is 4. Add them. 5 plus 1 plus 4 is 10. Divide by 7 and the remainder is 3. That is Tuesday.
Now take 17 May 1996. Date 17, May is 2, year code 1. The total is 20. Divide by 7 and the remainder is 6. That is Friday.
Work through one more at home. In the book, I use 18 January 1999. The total is 23, remainder 2, so it is Monday.
Why this makes a good practice
Many of us fall back on the phone or the wall calendar for such a thing. You will still use them. But when you do this sum in your head, your mind learns to hold small numbers and use them quickly. That is a skill for exams too.
Start with birthdays in your own family. Your mother's, your father's, your own. Then wedding days. Then the day your school started. Each answer you check against a real calendar tells you if you are right, and that feedback makes the codes stick.
Check yourself
Do the sum once for your own birthday, then look at a real calendar. If the day matches, you have understood the whole method. If it does not, check three things in this order. Did you add the month code correctly? Is it a leap year and a January or February date? Did you read the remainder against the right weekday? Almost every mistake hides in one of these three.
Do not rush
I will be frank. This needs practice. The first few dates will take you a full minute. Do five a day for two weeks and you will find the month codes arrive by themselves, and the sum takes seconds.
The book also gives codes for months and days of the week, such as April as April Fool and Tuesday as toast, to help when you need to remember an event by its month. Pick pictures that are personal to you. That is always better than mine.
This is one idea from my book Memory Power.
