Pi Day is an annual celebration of the mathematical constant π recognised primarily in the U.S.A. where they use a stupid date format. Pi Day is observed on 14 March since 3, 1, and 4 are the first three significant digits of π. Wikipedia

Celebrations: Pie eating, discussions about π
Date: March 14

Frequency: annual

Significance: 3, 1, and 4 are the three most significant figures of π

Related to: Pi Approximation Day