Pi Day π (pi) is observed on March 14 (3/14 in the month/day format) since 3, 1, and 4 are the first three significant digits of π. Pi (π) Day has become an international holiday, celebrated live and online all around the world. 

What is π anyway? Divide any circle’s circumference by its diameter; the answer (whether for a plate or a planet) is always approximately 3.14, a number represented with the Greek letter π. Mathematicians have been calculating π’s digits with more and more accuracy and have discover they go on literally forever, with no pattern.