Once a century, a very special day comes along. That day is today — 9/16/25.
Pi Day (3/14) often comes with sweet treats; Square Root Day (4/4/16 or 5/5/25, for example) has a certain numerical rhyme. But the particular string of numbers in today's date may be especially delightful to the brains of mathematicians and the casual nerds among us.
First, "all three of the entries in that date are perfect squares — and what I mean by that is 9 is equal to 32, 16 is equal to 42, and 25 is equal to 52," says Colin Adams, a mathematician at Williams College who was first tipped off about today's special qualities during a meeting with his former student, Jake Malarkey.
Next, those perfect squares come from consecutive numbers — three, four, and five.
But perhaps most special of all is that three, four, and five are an example of what's called a Pythagorean triple.
"And what that means," explains Adams, "is that if I take the sum of the squares of the first two numbers, 32 + 42, which is 9 + 16… is equal to 25, which is 52, so 32 + 42 = 52."
This is the Pythagorean Theorem: a2 + b2 = c2. "And that in fact is the most famous theorem in all of mathematics," says Adams.
It's a theorem that means something geometrically, too. Any Pythagorean triple — including 3, 4, and 5 — also gives the lengths of the three sides of a right triangle. That is, the squares of the two shorter lengths add up to the square of the final, longer side (the hypotenuse).
There are no other dates this century that meet all these conditions, so most of us will experience it just once in our lifetime.
(Fun bonus: It turns out the full year, 2025, is also a perfect square: 45 times 45.)
In any case, Adams says that if it were up to him, he'd call the day Pythagorean Triple Square Day. And he plans on celebrating with a rectangular cake cut along the diagonal to yield two right triangles.
"If I have any luck at all, if I can find a cake with the right dimensions, it'll look like a 3, 4, 5 cake, namely edge length 3, edge length 4, and edge length 5," he says. In the middle, he intends to have the date inscribed in icing.
"This date is hiding one of the most beautiful coincidences we will ever encounter," says Terrence Blackman, chair of the mathematics department at Medgar Evers College in the City University of New York. "Those numbers, they tell a story that goes back to ancient Greece."
Blackman says the Pythagorean Theorem is used frequently by carpenters and architects. But for him, as a mathematician, today's date captures a special elegance.
"It reveals some kind of hidden mathematical poetry that is sitting there — just like walking and coming upon a beautiful flower," he says.
In a world that can feel chaotic, Blackman feels that a day like today shows that math can provide a source of comfort.
"It reminds us that beauty and meaning can be found anywhere and everywhere," he says. "We just have to continue to look for it."
Copyright 2025 NPR
