For geeks, there are several great holidays on the calendar. There is of course Mole Day (10/23) to commemorate Avogadro's number, which is huge (on the order of 1023) and hugely important in physics. There's e Day (2/7) for Euler's ubiquitous number (e = 2.718…). But the best is Pi Day, held on March 14 because the infinitely long decimal approximation of pi begins with 3.14. There's so much to say about pi—I've been writing Pi Day posts for 14 years. (Here's a partial list).
What is pi (or as the Greeks would say, π)? By definition, it's the ratio of the circumference to the diameter of a circle. It's not obvious why that should be special, but pi shows up in a bunch of cool places that seem to have nothing to do with circles. But one of the weirdest things about pi is that it's an irrational number. That means it's a value that can't be expressed as a fraction of two integers. Oh, sure. The number 22/7 (22 ÷ 7) is a fair approximation, but it's not pi.