Pi is a very special number that you get if you divide the circumference of any circle by its diameter.
It is the same for all circles.
The number is 3.14159...etc....it just goes on and on...we usually just say Pi is 3.14.Usually in math we write Pi with the Greek letter π, which is the letter "p" in Greek.
Physicist Larry Shaw created Pi Day in 1988, with a public parade where his staff ate fruit pies and marched in a circle.
On Pi Day in 2004, Daniel Tammet recited 2,964 numbers in pi.
Then on March 12, 2009, the U.S. House of Representatives passed a resolution recognizing March 14 (3/14) as National Pi Day.