The Pythagorean Theorem tells us that, for a right triangle, the sum of the squares of the two short sides equals the hypotenuse squared.
What that looks like algebraically is:
a^2 + b^2 = c^2
What makes today, December 16th, 2020 Pythagorean Theorem Day? Well, plugging todays date into the Pythagorean equation gives us:
12^2 + 16^2 = 20^2
And thus, today is Pythagorean Theorem Day!