What's Pi Day you might ask? It is a day celbrated by many a geek around the world, all for that special mathematical constant π which equals 3.1415926.... Pi continues indefinitely, and so far it has been tracked out to 1,241,100,000,000 (over 1 trillion) digits after the decimal place using a computer.
Check out the official Pi Day website at http://www.piday.org/ or the Wikipedia explanation about Pi Day at http://en.wikipedia.org/wiki/Pi_Day
Need some suggestions how to celbreate Pi Day? Check out this Wiki How To article!