No, no, no, I’m not talking about the SCTV soap opera The Days of the Week although it’s pretty funny.

Quick, what day of the week is August 24, 2026? Monday!

Er, yeah, it’s today, I kind of knew that, but want if you got a slightly harder question? What day is October 12, 2026? Uh….

There are patterns, and you can do a little simple arithmetic in your head and figure out the day of the week for almost any date. Practice, and it’ll just take a few seconds! Here’s how! An algorithm by the famous mathematician John Horton Conway gives us a nifty way!

Here’s how Conway’s Doomsday Rule goes -

There’s a special day for the year, called the Doomsday, and for 2026, it happens to be Saturday. You memorize this tidbit.

You also memorize a few days that are always the same day as the Doomsday, one for each month, and you work forward and backward from there. For instance, 4/4, 6/6, 8/8, 10/10 and 12/12 are always the same day of the week - and in 2026, they’re all Saturdays. So October 12th 2026, well, it’s two days after 10/10 and we know that 10/10 is a Saturday, so October 12 is a Monday!

i can feel people closing this window and moving on to something else

Wait, stay with me!

You Can Work It Out.

The famous mathematician John Horton Conway figured out something he called the Doomsday Rule in 1973, and it’s a neat trick anybody can learn for figuring out days of the week.

You already know a little about how calendars work and days of the week (I hope.) There are 7, right? And 7 days from now is the same day as today, right? And 14 days from now? And 28 days from now? And 7 days ago?

You’re doing what mathematicians would call modulo 7 arithmetic, which is all about multiples of 7 and remainders, but let’s not get too hung up on that.

If you know that, say, August 8 is a Saturday, then … August 15 is also a Saturday, right? It’s a week later. And so is August 22 and August 29. All Saturdays. And August 1, if you go backwards. You’re adding and subtracting 7.

But what about August 17? Well … if August 8 is a Saturday, then August 17 is, um, 9 days later so that’s a week and two days so it must be a Monday.

I think most people are comfortable going backwards and forwards by sevens like this.

what if I told you you could use this trick to figure out any day in 2026?

Well, check this out.

All of these dates - every single year - will always fall on the same day of the week.

• 4/4 (April 4)
• 6/6 (June 6)
• 8/8 (August 8)
• 10/10 (October 10)
• 12/12 (December 12.)

See the pattern? 4/4, 6/6, 8/8, 10/10, 12/12. They’re all the same day of the week, every single year, and this year, 2026, they’re all Saturday.

We call this the Doomsday for 2026. All of those dates are Saturday, in 2026. Saturday is Doomsday, this year. (It’s different in other years, we’ll get to that.)

So. OK. Right now. What day is Christmas? Well, Dec. 25 is 13 days after 12/12, which was a Saturday, so that’s a week and six days so … Sunday!

What day is April 7th? Well, it’s three days after 4/4, which was a Saturday, so it’s …. Tuesday!

What about October 8th? Well, that’s two days before 10/10, which was a Saturday, so it’s …. Thursday!

See how this works? If we know that certain days of the year are Saturday, we can go backwards and forwards from that date to figure out some others.

4/4 6/6 8/8 10/10 12/12

That’s an easy pattern to remember. What about the other months?

ok what about the other months

Well the calendar is a bit of a mess with 28 days, sometimes 28, 30, 31 day months. We’re lucky that the 4/4 6/6 8/8 10/10 12/12 pattern just happens to work. What about the other months? What can we remember?

• 3/14 (March 14 - Pi Day!)
• 5/9 and 9/5 (May 9 and September 5)
• 7/11 and 11/7 (July 11 and November 7)

Those are all doomsdays too, so those dates, in 2026, are also Saturdays.
And you can remember Pi Day, the big math nerd day of the year, right?

And you can remember a phrase like I work from 9-5 at the 7/11. That reminds you that 9/5, 5/9, 7/11, and 11/7 are also Doomsdays, Saturdays.

Now we’ve almost, almost, got a Doomsday for every month, and knowing that Doomsday for 2026 is Saturday, we can almost do the entire year.

March 17? Three days after Pi Day (3/14), so three days after Saturday, so … Tuesday! November 6? One before 11/7, which is a Saturday, so … Friday!

Our list of Doomsdays is almost complete.

• 3/14
• 4/4
• 5/9
• 6/6
• 7/11
• 8/8
• 9/5
• 10/10
• 11/7
• 12/12

All of those, in 2026, are Doomsdays - Saturdays. (All of them, in 2027, are going to be Sundays, but we’ll get to that.) We can figure out most of the 2026 calendar by going backwards and forwards from the month’s Doomsday.

October 3? Uh. a week before October 10, which is a Doomsday Saturday, so … Saturday!

See? That’s easy

why are you avoiding January and February

I’ll get there. Look, here’s a calendar for 2026. Notice all those Doomsdays - in red here - how they’re all Saturday?

What about a different year, say … 1990?

Those special dates are all the same day of the week, but in this case, Wednesday. So we say The doomsday for 1990 was Wednesday.

(Nobody expects you to remember that, there’s a formula I’ll get to, but for now, let’s all agree that those magic Doomsday dates are always on the same day of the week.

Conway’s big observation

Conway’s big observation here was that if you learn some special dates, and learn what the Doomsday is for a particular year, it’s easy to figure out the day of the week.

May 14 1990? Well … let’s see .. the doomsday for 1990 was Wednesday, so we know 5/9 was a Wednesday, so 5/14 is five days later … Monday!

And so on.

Please hang on for a bit and I’ll explain how you figure out that the Doomsday for 2026 is Saturday, or the Doomsday for 1990 was Wednesday.

Quit stalling, what about January and February?

Leap years mess this up a bit because February now has 29 days. All the Doomsdays you learned still work in Leap Years, but for February? Here’s the rule:

The last day of February is always a Doomsday.

In 2026, February 28 is a Doomsday (so it’s Saturday.)
In 2024—a leap year— February 28 is a Doomsday.

The math still checks out whether it’s a leap year or not. So here’s your updated list:

• Last day of February (2/28 most years, 2/29 in a leap year.)
• 3/14
• 4/4
• 5/9
• 6/6
• 7/11
• 8/8
• 9/5
• 10/10
• 11/7
• 12/12

So February 9th, 2026 is … let’s see, Feb 28 2026 is a Doomsday so it’s Saturday, and if I go back three weeks, Feb 7 is also a Saturday, so Feb 9 is a Monday.

and finally, January

For January, Doomsday is the 3rd (if not a leap year) or the 4th (if a leap year.)

This would be your doomsday calendar for 2024 (a leap year) Doomsdays in 2024 are Thursdays, including the 4th of January, and the last day, the 29th, of February.

In a non-leap year like 2026, it’d look like this — the Doomsdays include January 3, and February 28

Finally, your complete Doomsday List in easy to remember form

• 4/4, 6/6, 8/8, 10/10, 12/12
• 5/9 and 9/5
• 11/7 and 7/11
• Pi Day 3/14
• Last day of February
• Jan 3 (non-leap-year), Jan 4 (leap year)

Some people even memorize a few more. US Independence Day (July 4) and Boxing Day (December 26) also happen to be Doomsdays.

Recap. How to do this JUST FOR 2026

Someone says a date. March 16. How do you think through this?

Step 0. You have to memorize that Doomsday List, but it shouldn’t be that hard, there are some nice patterns in there.

Step 1. Trust me, the Doomsday for 2026 is SATURDAY.

Step 2. We go through the list we memorized. What’s the doomsday for March? It’s 3/14 (Pi Day.) So that’s a SATURDAY.

Step 3. The 16th is 2 days later - so it’s MONDAY.

OK, what about …. August 5th? The Doomsday for August is 8/8, a SATURDAY. this is 3 days earlier so it’s a WEDNESDAY.

Won’t your friends be impressed?

Now, the big problem. How do you know the Doomsday for 2026 is Saturday?

If you want to be able to do this for ANY year, it’s a little tricker. You could just memorize a few, maybe going forwards and backwards from 2026….

• 2024 Thursday
• 2025 Friday
• 2026 Saturday
• 2027 Sunday
• 2028 Tuesday

as it advances by one day every year, but two days in Leap Year, but you can’t memorize them all.

Conway gives us a way, though.

more to come…