Happy Pi Day! Math fanatics from all around the world and everyone else who just loves pi (or pie) celebrate Pi Day on March 14.

Pi was named for the Greek symbol π, the ratio of the circumference of a circle to its diameter. The number’s first three digits are 3.14.

Pi Day is observed on March 14, since 3, 1 and 4 are the three most significant digits of π in the decimal form. On March 12, 2009, the U.S. House of Representatives passed a non-binding resolution recognizing March 14, 2009 as National Pi Day.

The first accurate calculation of π was conducted by Archimedes of Syracuse around 250 BC. The famous mathematician used the Pythagorean Theorem to find that the number was between 3 1/7 and 3 10/71. The first 15 digits of the never-ending number are: 3.14159265358979.

The earliest known use of the Greek letter π to represent the ratio of a circle’s circumference to its diameter was by mathematician William Jones in his 1706 work Synopsis Palmariorum Matheseos; or, a New Introduction to the Mathematics. The Greek letter first appears there in the phrase “1/2 Periphery (π)” in the discussion of a circle with radius one. Jones may have chosen π because it was the first letter in the Greek spelling of the word periphery.

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 celebrations for the day.

There are many ways of observing Pi Day, from eating pie and discussing the significance of the number π, to screening the film Life of Pi.

The Massachusetts Institute of Technology has often mailed its application decision letters to prospective students for delivery on March 14. 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.