It's March the 14th. Yep, 3.14. π day!

π is the ratio between a circle's circumference and its diameter. It's an irrational number, which means its digits go on forever with no repeating pattern. So if you were to look hard enough, you can find the complete works of Shakespeare encoded in the digits. (Of course, that would be a long way in. And since the digits are infinte, you'll find an infinte number of copies of Shakespeare hidden in there, at unpredicatable locations! (Infinites are fun!))

This handy little number can be found in all sorts of equations. From simple things like relating areas and volumes of circles and spheres and other curved objects to their thickness (like A=πr²), to big things like gravitation (Einstein's field equations for general relativity: G_μν = 8πGT_μν/c⁴), to tiny things like quantum mechanics (Heisenberg's uncertainty principle: ΔxΔp >= h/4π).

It's also a part of one of the most beautiful equations ever discovered, Euler's identity: e^iπ + 1 = 0. I call this equation beautiful because it non-trivially relates 8 of the most fundamental components of mathematics:
Addition
0, the additive identity
Multiplication
1, the multiplicative identity
Exponentiation
e, the exponential differentiation identity
i, the imaginary unit
and π.
How could something that relates all these things so elegantly not be beautiful?

These days we know far, far more digits of π than are practically necessary. You need less than 40 digits to be able to compute the size of a circle as big as the observable universe to the precision of a hydrogen atom. And yet we know π up to 1.2 trillion digits. At this point, it's just for the fun!

So, with that in mind I hope you have a fun π Day!

3.1415926535897932384626433832795...