March 14 is Pi Day. It is a day to celebrate the mathematical constant pi (π) and to eat lots of pie. One of the oldest and the most recognizable mathematical constant in the world, Pi (π) is the ratio of any circle’s circumference to its diameter. Its value is approximately equal to 3.14159265. It is an irrational number, which means it cannot be expressed as a ratio of whole numbers, and its decimal representation never ends or repeats.