Pi Day (March 14) is a day of global mathematical celebration, but it’s not the only numerically significant calendar date. It’s far from the rarest, either. In fact, today marks a special occasion that only occurs once this century. Not only is each number in today’s date (9/16/25) a perfect square—their consecutive square roots are also an example of a Pythagorean triple. While an official name has yet to be assigned, “Pythagorean Triple Square Day” encapsulates the moment pretty perfectly.
Here’s the specific math to add it all up: 3 multiplied by itself is 9, 4 squared is 16, and 5 squared is 2025. On top of that, the sum of the first two square roots adds up to 25.
“This date is hiding one of the most beautiful coincidences we will ever encounter,” Terrence Blackman, head of mathematics at Medgar Evers College in the City University of New York, told NPR. “Those numbers, they tell a story that goes back to ancient Greece.”
To be fair, the story actually goes even further back than that. Pythagoras (c. 570—c. 495 BCE) is most famously credited as the first mathematician to discover his namesake theorem. To jog those scary memories of high school math, the Pythagorean Theorem states that in a right-angled triangle, the square of the hypotenuse is equal to the sum of the squares of the two other sides.
However, both ancient Babylonian and Indian theorists had known the same information for centuries. Additional accounts also ascribe him as being one of the first Greek thinkers to teach that the world was spherical, although that credit is more likely owed to Parmenides.
But regardless of authorship, the Pythagorean Theorem helped revolutionize modern concepts in math, geometry, architecture, music, and numerous other fields. The next Pythagorean Triple Square Day won’t take place for quite some time. However, to make things extra special, today’s date possesses its own mathematical elegance: just type 45 squared into a calculator to find out why.
