Doomsday Algorithm Trainer

This game challenges you to determine the correct day of the week for a randomly selected date. Read on (or alternatively watch this video) to learn the Doomsday Algorithm, created by my favourite mathematician, John Conway.

Conway noticed that these dates always fall on the same weekday each year:

He called these days 'Doomsdays'. If you know the Doomsday of a particular year, you can simply count forwards or backwards from one of these known dates to find the one you want! The Doomsday for 2026 is SATURDAY.

To find the Doomsday for any year:

1. Take the last two digits of the year
2. Divide by 12 → keep the quotient
3. Keep the remainder
4. Divide the remainder by 4 → keep the quotient
5. Add the three numbers
6. Add that to the century anchor day

Century anchors:
1800s = Friday
1900s = Wednesday
2000s = Tuesday

E.g. 11th January 1942... 42 divided by 12 is 3 remainder 6. 6 divided by 4 is 1. So 3 + 6 + 1 = 10. Add 10 days onto the 1900's anchor day of Wednesday. This gives a 1942 Doomsday of Saturday! And the 11th January is 8 days more than 3/1, therefore the day must be SUNDAY.

One last thing is that you need to be able to distinguish leap years. The rule is if the last two digits of the year are divisible by 4, then it is a leap year. EXCEPT if it is a century year e.g. 1800 or 1900, it is only a leap year if it is divisble by 400. So 1900 was NOT a leap year, but 2000 was.