Entrance mathematician’s building, TU-Berlin

Pi (π)
The number π is a mathematical constant, the ratio of a circle’s circumference to its diameter, approximately equal to 3.14159(The first 100 decimal digits are 3.1415926535897932384626433832795028841971693993751058209749445923078164062862089986280348253421170679).

Pi Day
Pi Day is an annual celebration commemorating the mathematical constant π (pi). Pi Day is observed on March 14 (or 3/14 in the U.S. month/day date format), since 3, 1, and 4 are the three most significant digits of π in the decimal form. In 2009, the United States House of Representatives supported the designation of Pi Day.
Pi Approximation Day is observed on July 22 (or 22/7 in the day/month date format), since the fraction 22⁄7 is a common approximation of π, which is accurate to two decimal places and dates from Archimedes

History
The earliest known official or large-scale celebration of Pi Day was organized by Larry Shaw in 1988 at the San Francisco Exploratorium, where Shaw worked as a physicist, with staff and public marching around one of its circular spaces, then consuming fruit pies. The Exploratorium continues to hold Pi Day celebrations.
On March 12, 2009, the U.S. House of Representatives passed a non-binding resolution (HRES 224), recognizing March 14, 2009, as National Pi Day.
For Pi Day 2010, Google presented a Google Doodle celebrating the holiday, with the word Google laid over images of circles and pi symbols.
The entire month of March 2014 (3/14) was observed by some (University of Maryland Alumnus) as “Pi Month”.
In the year 2015, Pi Day will have special significance on 3/14/15 at 9:26:53 a.m. and p.m., with the date and time representing the first 10 digits of π. That same second will also contain a precise instant corresponding to all of the digits of π. However, some argue that 9:26:54 a.m. and p.m. on 3/14/15 are more accurate because of the 11th digit of π being 5, which would cause the 10th digit to round up to 4.

Observance
Pi Day has been observed in many ways, from eating pie to discussing the significance of the number π. Some schools hold competitions as to which student can recall Pi to the highest number of decimal places.

The Massachusetts Institute of Technology (MIT) has often mailed its application decision letters to prospective students for delivery on Pi Day. Starting in 2012, MIT has announced it will post those decisions (privately) online on Pi Day at exactly 6:28 pm, which they have called “Tau Time”, to honor the rival numbers Pi and Tau equally.

The town of Princeton, New Jersey, hosts numerous events in a combined celebration of Pi Day and Albert Einstein’s birthday, which is also March 14. Einstein lived in Princeton for more than twenty years while working at the Institute for Advanced Study. In addition to pie eating and recitation contests, there is an annual Einstein look-alike contest.

Source
Wikipedia.