Pi day is a celebration of the mathematical constant for pi. Pi day is observed on March 14th ( 3/14 in the month/day date format).
The earliest known official or large scale celebration of Pi Day was organised by Larry Shaw in 1988 in San Francisco Exploratorium where Shaw worked as a physicist, with staff and public marching around one of its circular spaces, and then consuming fruit pies.
In 2015 Pi Day had special significance on 03/14/15 at 9:26:53am and also at pm, with the date and time representing the first 10 digits of Pi.
In 1996 Pi Day was also significant because its mm/dd/yy represented pi rounded to the first five digits.
Taken from Wikipedia page for Pi Day