Pi (π) day Happy Pi Day! Pi Day is celebrated on March 14th (3/14) every year to honor the mathematical constant pi (π), which represents the ratio of the circumference of a circle to its diameter. Pi is an irrational number, which means it cannot be expressed as a finite decimal or fraction. Pi has been known for thousands of years, and its value has been calculated to trillions of digits. Although it is commonly approximated to 3.14, it is actually an infinitely long decimal number that never repeats. Pi is used in many areas of mathematics and science, including geometry, trigonometry, calculus, and physics. Pi Day is celebrated in many ways, such as by eating pie, participating in math competitions, and reciting the digits of pi. It is a fun and educational way to appreciate the beauty and complexity of mathematics.