Pi Day is an annual celebration of the mathematical constant π (pi). Pi Day is observed on March 14 (the 3rd month) since 3, 1, and 4 are the first three significant figures of π.
St. Patrick's Day—observed every March 17—is packed with parades, good luck charms, and all things green. The event started as a religious holiday, but over time it's become a celebration of Irish culture.