# Pi Day

There are two days held in honor of the mathematical constant π (Pi): Pi Day and Pi Approximation Day.

## Pi Day

March 14, written 3-14 in the USA date format, is an unofficial celebration for Pi Day derived from the common three-digit approximation for the number π: 3.14. It is usually celebrated at 1:59 PM (in recognition of the six-digit approximation: 3.14159). Some, using a twenty-four-hour clock rather than a twelve hour clock, say that 1:59 PM is actually 13:59 and celebrate it at 1:59 AM or 3:09 PM (15:09) instead. Parties have been held by the mathematics departments of various schools around the world.

This day has been celebrated in a variety of ways. Groups of people, typically pi clubs, give thought to the role that the number π has played in their lives and imagine the world without π. During such an event, pi celebrants may devise alternative values for π, eat pi (pie), play pi (piñata), drink pi (Piña Colada) or watch π (Pi (film)). Enthusiasts also note that the day happens to be Albert Einstein's birthday.

The "ultimate" pi day[1] occurred on March 14th, 1592, at 6:53 AM and 59 seconds. When written in American-style date format, this is 3/14/1592 6:53.59, which corresponds to the first twelve digits of pi: 3.14159265359 (rounded of course). However, considering this was well before any kind of standardized world time had been established, and the general public had no concept of π, the occurrence likely went unnoticed[2].

## Pi Approximation Day

Pi Approximation Day is one of two days: either July 22 (written 22/7 — in some date formats — 22 divided by 7 is an approximation to π), or April 26 (April 25 on leap years), the day on which planet Earth completes two Astronomical units' worth of its annual orbit: on this day the total length of Earth's orbit, divided by the length already traveled, equals π (that is, the Earth has travelled two radians in its orbit).