Pi Day is celebrated on Mar 14th every year as the month/date format reads 3/14, where the number 3.14 is commonly shortened as the value of pie (3.14159265~). Pi is a mathematical constant whose value is the ratio of any Euclidean plane circle's circumference to its diameter; this is the same value as the ratio of a circle's area to the square of its radius.
2010 Pi Day Google doodle showed various uses of π: measuring the volume of a sphere (V = 4⁄3 πr3), computing the circumference of a circle (C = 2πr), measuring the volume of a cylinder (V = πr2h), Archimedes" calculation of Pi (223/71 < π < 22/7), and the periodic function of sin(x).
Interestingly, Google featured a special doodle in Indian search engine home page to mark the 80th anniversary of first Indian movie with sound, Alam Ara. In this black and white Doodle, a pillar and the image of the girl replaces the fifth letter of the name Google - L.