from pi day dot org:

Pi, Greek letter (), is the symbol for the ratio of the circumference of a circle to its diameter. Pi Day is celebrated by math enthusiasts around the world on March 14th. Pi = 3.1415926535…

With the use of computers, Pi has been calculated to over 1 trillion digits past the decimal. Pi is an irrational and transcendental number meaning it will continue infinitely without repeating. The symbol for pi was first used in 1706 by William Jones, but was popular after it was adopted by the Swiss mathematician Leonhard Euler in 1737. Learn more about Pi.

There are many places out there celebrating.. including Underwired in Bethlehem, PA, where you can have pie if you arrive between 12 and 3.. and discounts on all round objects!