There are two days held in honour of Pi: Pi Day and Pi Approximation Day.
March 14 is the official day for Pi Day derived from the common three-digit approximation for the number Pi, 3.14. It is usually celebrated at 1:59 pm (3.14159). Parties have been held by mathematics departments of various schools around the world.
Pi Approximation Day is July 22. This date is written 22/7 in non-US date format, and the fraction 22/7 is a common approximation to pi.
This holiday, while considered silly to some, has been celebrated in a variety of ways over the years. Groups of people, typically pi clubs, give thought to the role that the number pi has played in their lives and imagine the world without pi. During the event, pi celebrants devise alternative values for pi, eat pi cake and often play pi pinata.
See also: Mole Day, Programmer's day, A simple proof that 22/7 exceeds &piPi Day
Pi Approximation Day