Your calendar just aced a geometry quiz. Happy Pythagorean Triple Day: today, Oct. 24, 2026, is the last date this century where the month squared plus the day squared equals the two-digit year squared.
That makes it a Pythagorean triple day nobody’s calendar will repeat until 2105.
Here’s the math, and I promise it’s painless. Write today as 10/24/26. Then 10 squared is 100, and 24 squared is 576. Add them up and you get 676, which is exactly 26 squared.
A perfect right triangle, hiding in plain sight on a Saturday.
Why This Pythagorean Triple Day Is the Last One
The viral reminder comes from Not Math Club. The account describes itself as American University’s club for talking about the life of mathematics. Its Instagram caption, as shown in the screenshot of the post, says it’s “the final time this century” the pattern works. I couldn’t pull up the original post myself, so I can’t vouch for its engagement numbers. The math, though, holds up.
I ran the numbers on every valid month, day and two-digit-year combination from 2001 to 2099. Only a short list qualifies. It runs 3/4/05, 6/8/10, 5/12/13, 9/12/15, 8/15/17, 12/16/20, 7/24/25 and today’s 10/24/26 (plus a few flipped versions like 4/3/05).
Nothing after today works. That’s my own computation, not a published source, but you can check it on the back of a napkin.
Math educators agree on the big picture. Oxford’s maths department is marking Pythagorean Theorem Day and confirms the date fits the pattern. Writer Adam Spencer, in an Oct. 23 post, says there won’t be another this century.
The date also works either way you write it. Use 10-24-26 in US format, or 24-10-26 if you put the day first.
Here’s the Thing: It’s a Fake Holiday With Real Math
Let’s be honest about what this is. It’s an informal, unofficial math holiday. Nobody is getting the day off, and your bank is not closing.
Some fans even grumble that 10/24/26 is a bit of a cheat. After all, 10, 24 and 26 are just double the classic 5, 12, 13 triple.
So, a repeat of a repeat. Still, a right triangle you can celebrate with a calendar is rare. This is the last chance for a while.
If you want to get in on the triple square day fun, pick three numbers and test them. The classic is 3, 4, 5, because 9 plus 16 makes 25. For a harder swing, try 5, 12, 13 or 8, 15, 17.
Teachers and bloggers are already sharing triangle activities for classrooms and kitchen tables. There’s no shortage of ways to make a kid groan and then grin. Parents already juggling the rising cost of back to school may appreciate a free lesson. And if the kids are logging attendance, here is why showing up matters for school funding.
When Is the Next Pythagorean Triple Date?
Don’t mark your calendar just yet, because the wait is long. One unofficial source says the next qualifying date is April 3, 2105. It fits the same arithmetic: 4 squared plus 3 squared equals 5 squared, or 04/03/05. That’s a single, unofficial source, though the math checks out.
Which means the next generation of math nerds will get their turn long after everyone reading this is gone. Meanwhile, if you like numbers with real stakes, see how a 7.28% mortgage rate is outpacing falling home prices.
Enjoy the triangle while you’ve got it, and tell someone it’s a good day to be a square.
