Simply put, pi is a mathematical constant that expresses the ratio of a circle’s circumference to its diameter. It figures into numerous formulas used in physics, astronomy, engineering and other fields, dating back thousands of years to ancient Egypt, Babylon and China.

On March 14, math classes across the country will celebrate Pi Day—a national holiday in honor of the mathematical constant pi, which represents the ratio of a circle’s circumference to its diameter and starts with the digits 3.

March 14 is Pi Day in honor of the first three digits of the mathematical constant π (3.1415…). But it’s also a flimsy, yet delicious, excuse to celebrate the many edible varieties of pie around the world.