Pi Day is an annual celebration commemorating the mathematical constant π. Pi Day is observed on March 14, since 3, 1, and 4 are the three most significant digits of π in the decimal form.

Today, millions of people worldwide will celebrate both a mathematical constant and a sweet filling in a wheaten crust. In other words—Pi Day.

Pi is the ratio used to compute the circumference, area, and volume of circles.

Pi Day gets extra credit because it’s also Albert Einstein’s birthday. The German theoretical physicist was born on March 14, 1879.