Doomsdays

Doomsdays

The fact that certain dates land on the same weekday as each other every year is the first principle to understand when mastering the Doomsday algorithm. What we need next is a system to help us memorise a set of key dates, spread through the year, that all land on that same weekday.

The dates we are about to learn are called doomsdays.

In all the even months, apart from the special month of February, the doomsday is the same as the month number.

Remembering the phrase I work at 7-Eleven from 9 to 5 makes four more doomsdays easy to recall. The numbers in it — 9, 5, 7 and 11 — pair up as month and day in both directions: 9/5 and 5/9, 7/11 and 11/7.

The remaining three months are the first three of the year:

The doomsday in January depends on whether the year is a leap year. In the three common years of the cycle it is the 3rd, and in the leap year itself it is the 4th.

In February, we simply need to remember that the last day of the month is a doomsday. In a common year that is the 28th; in a leap year it is the 29th.

Since February is the month leading up to March, some people find it easier to remember the doomsday in February as the 0th day of March. This is the day before the first day. It also means you can use the 7th, 14th, 21st and 28th — the multiples of 7 — as doomsdays in March.

Here you can see that all the doomsdays are Thursdays in 2024:

The initial challenge in determining a weekday for a specific date lies in identifying the doomsday for that year. After establishing the year’s doomsday, you can then easily calculate the desired date’s weekday by counting forward or backward from the nearest doomsday.