13 Doomsdays

Tomorrow, December 26th, is a Doomsday... 

Many years ago, I read a book on the calendar, and there was a chapter on Lewis Carroll's method of calculating the day of the week for any date in any year. Simplifying that idea, British mathematician John Conway gave us the Doomsday Algorithm. 

Every year has its Doomsday. It's a day of the week. This year (2025) it's Friday. Next year (2026) it's Saturday. (Which makes things easy as it moves forward one day every year except for leap years when it's two.) 

Certain days fall on the Doomsday. Remember them, and you can count from them to figure out other days. And people think you're way smarter than you actually are. 

So, today for Thursday 13, thirteen days that fall on the Doomsday.

1. December 26th

Or Boxing Day in other parts of the world. Which makes it easy to know what day of the week Christmas is going to fall on in any given year as well as the following January 1st, as that is exactly a week after. 

2. March 0

Or the last day of February. This year that was February 28th. Next year that will be February 28th. But in a leap year it'll be February 29th. And this is the easiest way to note that. 

3. March 28th

This one I remember because non-leap year February and March are the same month. Basically. 

There are seven days in the week. Each month starts on one of those days. But there are twelve months. If two months start on the same day of the week (and mathematically, some have to), then I call them the "same month". That is, all the days will fall on the same day of the week. (Well, except for end of the month, as February only has the 28 days while March has 31.) 

4. November 28th

Because November is the same month as March and non-leap year February. Don't believe me? Pull out a calendar. 

5. October 31st

Halloween. Not the only holiday to fall on the Doomsday (see #1). 

6. January 31st*

Because October and January (non-leap year) are the same month. (And both have 31 days, which makes them exactly the same month.) Feel free to check your calendar...

7. April 4th

There's a funny thing with even (non-February) months. The month/day when it's the same, is the Doomsday. As April is the fourth month (4/4)... 

8. July 4th

And July and April are the same month. Every year. 

9. June 6th

So, continuing from #7, this is 6/6. It's a nice thing to remember, as it gives most months one day you can remember as the Doomsday to make the calculations easier. 

10. August 8th

Yup, 8/8.

11. October 10th

And that gives us 10/10.

12. December 12th

Finally, 12/12. Add seven, and the 19th is the Doomsday. Seven more gets you 26 (see #1). 

13. September 12th

And finally, September and December are the same month. 

Did you know these? Do you now think I know way too much about the calendar?