Here’s a cool party trick: being able to tell the weekday that corresponds to the given. If you want to impress your friends and family, then you can learn it using the doomsday algorithm, invented by Jhon Conway. It is surprisingly simple and only takes 30 minutes to learn.

To calculate the weekday for any date, you are going to start assigning numbers to days of the week. I like to give them little nicknames.

Sunday = noonday = 1

Monday = oneday = 2

Tuesday = twosday = 3

Wednesday = threesday = 4

thursday= fourday = 5

friday= fiveday = 6

saturday= sixaday = 7

Then you are going to need to memorize the doomsdays. Write these down to secure them into your memory. These are important because all of these dates land on the same day of the week as each other. For example, if the fourth of April is Monday in any particular year then you know that the sixth of June is also a Monday.

Doomsdays:

4/4 = fourth of April

6/6 = Sixth of June

8/8 = eight of august

10/10 =tenth of October

12/12 =twelfth of December

9/5 = Ninth of may

5/9 = Fifth of September

7/11 = Seventh of November

11/7 = Eleventh of July

Doomsdays are a bit trickier when it comes to January, February, and March.

For January, our doomsday is the third for three years, and then it’s the fourth on the fourth year because of the leap year.

For February our doomsday is the last day of January, the 28th for the first three years, and then it’s the 29th for the fourth year.

For march, our doomsday is the 14th of March, also known as pi day.

Remember, all of these dates share the same day of the week as each other.

If the date you are looking for is not one of those special dates, then you would simply adjust for the date you’re looking for. For example, if you wanted to find the day of the week for the ninth of June, you would take the doomsday which is the 6/6, and then add three. Because in this case, the doomsday is Monday, then the ninth of June would be Thursday, three days ahead.

This is the simple formula to find the day of the week for any date within the year. But if you want to branch out from there, then check out this video from numberphile. He explains how to do it for any date in time, even hundreds of years back.

https://www.youtube.com/watch?v=z2x3SSBVGJU&t=0s