Pi Approximation Day is marked every year on July 22 to celebrate the mathematical constant pi (π). The date is chosen because, when written in the day/month format as 22/7, it matches the fraction most commonly used as an approximation of pi. Pi is the ratio of a circle's circumference to its diameter and is an irrational number with a value of about 3.14159. The day is used to spark interest in mathematics among students and the public. The theme for 2026 is "Mathematics and Hope".