Pie day: PI
From wikipedia: Pi Day and Pi Approximation Day are two holidays held to celebrate the mathematical constant π (pi) (in the mm/dd date notation: 3/14); since 3, 1 and 4 are the first three digits of π. March 14 is also the birthday of Albert Einstein and the two events are sometimes celebrated together.
Pi Approximation Day is observed on July 22, because of Archimedes' popular approximation of π being 22/7. However, this may be considered misleading, as all cited dates are "approximation days" (as π is an irrational number) and 22/7 is actually a closer approximation of π than 3.14 is. Typically, March 14 is more popular for countries using the month/day format and the 22nd of July is more popular for countries using the day/month format.
For more on this go wikipedia
How do we celebrate Pi day
Simple most common way of celebrating Pi day is eating pie
founder of Pi Day Larry Shaw still helps out with the celebrations.
Here is my lovely Pi day pie