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. Linda Gordon draws cartoons for the website that her husband Ron Gordon made to celebrate mathematically themed days.

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.

Even more interesting?

(Fun bonus: It turns out the full year, 2025, is also a perfect square: 45 times 45.)