Pi Day is celebrated around the world on March 14 because 3.14 are the first digits of pi. The symbol of the Greek letter “π” was first used by William Jones in 1706. The actual celebration starts at 1:59 PM so that the exact number 3.14159 can be reached. Pi (π) is the ratio of a circle’s circumference to its diameter. It is a constant number which means that for all circles of any size, Pi will remain the same. A ‘p’ was chosen for ‘perimeter’ of circles, and the use of π became popular after it was adopted by the Swiss mathematician Leonhard Euler in 1737. According to Piday.org, Pi has been calculated to over one trillion digits beyond its decimal point. The earliest known official or large-scale celebration of Pi Day was organized by Larry Shaw in 1988 at the San Francisco Exploratorium.