Pythagorean Theorem Day is celebrated when the sum of the squares of the month and day of a date equals the square of the last 2 digits of the year of that date. So we have one in December, 12/16/20 (122+162=202 or 144+256=400), another one on 7/24/25, and the last one this century will be 10/24/26. But that will be your last chance to celebrate until March of 2105, so don’t wait, whoop it up now!
In honor of the day (and the Theorem), I have included below, one of my favorite graphical proofs of the Pythagorean Theorem. Look closely at the animated image and you can see why, with a right triangle with sides a, b, and c, that a2+b2 must be the same as c2. Note that the right triangles remain the same, they merely get rearranged. It is the left over area that gets reallocated as a2+b2 or c2 within the larger fixed-size square.