Calculating Doomsday: not as dramatic as it sounds

A three-step program for figuring out what day of the week any date is.

If+you%E2%80%99re+looking+to+use+a+retro+calendar%2C+1977%2C+1983%2C+and+1994+all+start+on+a+Saturday+and+aren%E2%80%99t+leap+years%2C+just+like+2022.

If you’re looking to use a retro calendar, 1977, 1983, and 1994 all start on a Saturday and aren’t leap years, just like 2022.

Ella Whalen, Staff Writer

This may come as a surprise, but there are only 14 possible years relative to the days of the week that dates fall on. Ignoring holidays that shift by a different calendar (like some Jewish and Muslim ones), a year can only start on seven days, Monday to Sunday, and the only other factor is whether it’s a leap year. While not useful in day-to-day life, this enables a nifty mathematical trick known as the ‘Doomsday algorithm’.

There are specific days, the ‘Doomsdays’ of the algorithm, that always fall on the same day of the week compared to each other and are rather simple to remember. These days are 4/4, 6/6, 8/8, 10/10, 12/12, 9/5, 5/9, 7/11, 11/7, and 3/14. For instance, just recently was the 4th of April, which was a Monday; therefore, all these dates this year will be on a Monday as well. January and February are all impacted by the leap year, but they have ‘Doomsdays’ as well—1/3 and 2/28 when it’s not a leap year, or 1/4 and 2/29 when it is. Other dates fall on this day as well, such as the Fourth of July and Halloween, and those will work in the algorithm as well.

Once you know the ‘Doomsday’ for a particular year, getting a day for a date in that year is just a matter of adding weeks and days. Since adding subtracting seven to a date returns you to the same day, you do that as many times as possible until you’re close to the date, then it’s a matter of remembering the order of the days. For example, 4/4 was a Monday this year, so 4/11, 4/18, and 4/25 are as well. Adding the remaining five days until 4/30 tells us that April this year will end on a Saturday.

Determining the ‘Doomsday’ for a given year follows a pattern as well. Each successive year, ‘Doomsday’ moves forward by one day, except for leap years where it moves forward two days. For instance, ‘Doomsday’ this year is a Monday, so it will be a Tuesday in 2023 and a Thursday in 2024. Knowing a few base years comes in handy for this: 2000’s ‘Doomsday’ was a Tuesday, 1900’s was a Wednesday, 1800’s was a Friday, and 1700’s was a Sunday. This pattern repeats every 400 years, with 2100’s being a Sunday, 2200’s being a Friday, and so on. ‘Doomsday’ also repeats every 28 years, so years ending in 28, 56, and 84 have the same ‘Doomsday’ as their base year (e.g. 1984’s was a Wednesday as well).

If you’re having trouble remembering how long the months are, there’s a handy trick for that, too. Put your two fists side-by-side, with the backs of your hands towards you. Start counting with January as your pinky knuckle, February as the spot between your pinky and ring finger, March as your ring knuckle, and so on. Every month that lands on a knuckle has 31 days, and every month that doesn’t has fewer. (https://youtu.be/p6Ma0D-fN38)