Pi, Greek letter (
), is the symbol for the ratio of the circumference of a circle to its diameter. Pi = 3.1415926535… Pi Day is celebrated by math enthusiasts around the world on March 14th.
See wat official google blog has about the day
http://googleblog.blogspot.com/2009/03/magic-number.html
And finally wishing you Happy Pi Day and leaving you to official Pi day website