Pi represents the ratio of the circumference of a circle to its diameter.
Pi is a number that will continue infinitely without a pattern. It has been calculated to over 1-trillion digits beyond its decimal point! So...we just refer to it as 3.14, which is why celebrate it on March 14th (3/14).
What will you do to celebrate Pi Day? I suggest ordering a pizza...
Then, measure the circumference and diameter of the pizza. Divide the circumference by the diameter. While you calculate it out to, let's say, 50 digits... I'll eat the pizza. Good times.