The mathematical value of π, or pi, begins "3.14..." so March 14 (3/14) is considered to be International Pi Day.
Pi is the English spelling of the sixteenth letter of the Greek alphabet. It means "the symbol π denoting the ratio of the circumference of a circle to its diameter," and also means "the ratio itself : a transcendental number having a value rounded to eight decimal places of 3.14159265." This means that the outside measurement of a circle is a little more than three times its width. This ratio is one of the most important constants in mathematics.
Pi is probably used to stand for this number because it is the first letter of the Greek word that is the origin of periphery.