Pi Day is an annual celebration of the mathematical constant π (pi). Pi is the ratio of a circle's circumference to its diameter, and it is approximately equal to 3.14159. Pi Day is celebrated on March 14th (3/14) because the first three digits of pi are 3, 1, and 4.

Pi Day was first celebrated in 1988 by physicist Larry Shaw at the San Francisco Exploratorium. The Exploratorium has continued to host Pi Day celebrations every year since then, and Pi Day has become a popular holiday around the world.

There are many ways to celebrate Pi Day. Some people choose to eat pie, while others participate in math-related activities. Some schools and libraries hold Pi Day contests, and some businesses offer Pi Day discounts.

Here are some fun facts about pi:

- Pi is an irrational number, which means it cannot be expressed as a simple fraction.

- The digits of pi appear to be random, and there is no known pattern to them.

- Pi has been calculated to over 100 trillion digits, and there is no known limit to how many digits it has.

- Pi is used in many formulas in mathematics and science, including the formula for the area of a circle and the formula for the volume of a sphere.

- Pi has been known for thousands of years, and it was first calculated by the ancient Greeks.

Pi Day is a great day to learn more about mathematics and science, and it is a fun way to celebrate the beauty of math.