Why is March 14 (or 3/14) Pi day? Because Pi (pronounced "pie") is defined as the ratio of a circle's circumference to its diameter. And in our number system it's written out as:
3. 14159265358979323846264338327950288419716939937510...It's an irrational number, meaning that it can't be written out as a simple fraction (i.e. as a "ratio" of two integers a/b where b doesn't equal zero) as well as a transcendental number, meaning that it's not a root of a non-zero polynomial with rational coefficients.
And it goes on forever.
Happy Pi Day!