3.141592654. That's Pi (π), at least to the greatest number of digits I can recall. But when I was in school, frequently we would use 3.14. Thus you can see why today is Pi Day. It all adds up.
There's even an official Pi Day website. No kidding. For those of you not in the know, Pi is "the relationship between a circle’s diameter (its width) and its circumference (the distance around the circle)."
So, how can you celebrate Pi Day? Well, you can send a Happy Pi Day e-card, buy some Pi Day merchandise, or visit San Francisco's Exploratorium for their Pi Day celebration.
Of you can do what I'm going to do: head for Baker's Square and eat some pi(e).
Oh, and of course a more precise Pi Day will be in 2015. 3/14/15.