Pi Approximation Day is celebrated on July 22 due to popular approximation of pi of Archimedes’ 22 / 7. However, this can be misleading, since all the dates cited are days “approach” (as pi is an irrational number) and 22 / 7 is actually a little closer than 3.14 pi. Typically, March 14 is more popular in countries that use the format month / day (22 / 7 is a possible date in this format), and July 22 is more popular in countries using the format day / month (from 3 / 14 and 31 / 4 dates are impossible in this format).