July 22 Pi Approximation Day
Pi Day and Pi Approximation Day are two holidays held to celebrate the mathematical constant π (pi). Pi Day is observed on March 14 (in the American mm/dd date notation: 3/14); since 3, 1 and 4 are the first three digits of π.
Pi Approximation Day is observed on July 22 because of Archimedes’ popular approximation of π being 22/7. However, this is misleading, as all cited dates are “approximation days” (as π is an irrational number.)
But 22/7 is actually a closer approximation of π than 3.14. There are a large variety of ways of celebrating Pi Day and most of them include eating pie and discussing the relevance of π. Typically the 22nd of July is more popular for countries using the day/month format and March 14 is more popular for countries using the month/day format.