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.

This year is a big one for the math nerds out there — and for many, a once-in-a-lifetime occasion. That’s because 2025 is a “perfect square,” a number that’s the product of a squared integer — 45 ...