9-16-25 was a rare, once-in-a-century date. Did you notice?

Special dates, such as March 14 - 3/14 - (Pi Day), and May 4 (Star Wars Day) generate a lot of excitement every year. For mathematicians, there is also the Square Root Day (4/4/16 or 5/5/25). We just had another such special date - 9/16/25 (September 16, 2025). There is no ring to it, or you might not instantly associate it with any special theorem or mathematical figure. Yet, there was something about this date that fascinated maths fanatics. Colin Adams, a mathematician at Williams College, told NPR that he was first told about the speciality of the date by a former student named Jake Malarkey. He said, "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." But, that's not all. The other noticeable thing about the date is that all three numbers - 9, 16 and 25 - come from consecutive numbers — three, four, and five.

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Next, three, four, and five are examples of what's called a Pythagorean triple. Adams says the figures come together to form the "most famous theorem in all of mathematics" - the Pythagorean Theorem. The numbers form a2 + b2 = c2. He explains, "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." The Pythagorean Theorem is a2 + b2 = c2.

Pythagorean triple

In geometry, any Pythagorean triple 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. This is true for 3, 4 and 5 as well. Adams says this is a once-in-a-lifetime date as there are no other dates in this century that will meet all these conditions. More maths magic is dripping here, as the year 2025 is also a perfect square - 45 times 45.

Adams also has a name ready for this mathematically magical date. He says he would love to call it Pythagorean Triple Square Day. He said celebrations would include a rectangular cake cut along the diagonal to yield two right triangles. Blackman, completely in awe of the date, says it reveals "some kind of hidden mathematical poetry".