There’s absolutely nothing wrong with celebrating the little things. Though, let’s be honest, Pi is not a little thing at all. It’s a mathematical constant. It is an infinite decimal.
The modern symbol for Pi was first used (in our modern sense) in 1706 by William Jones:
There are various other ways of finding the Lengths or Areas of particular Curve Lines, or Planes, which may very much facilitate the Practice; as for instance, in the Circle, the Diameter is to the Circumference as 1 to (16/5 – 4/239) – 1/3(16/5^3 – 4/239^3) + … = 3.14159… = 
(A History of Mathematical Notation by Florian Cajori)
And, yes, you may also celebrate by eating a piece of pie. Just think about the fact that Pi is the ratio of the circumference of a circle to its diameter while you’re cutting a slice.
