Pi Day 2021: What is it and why it is celebrated on March 14? Published on March 14, 2021 Entertainment 1045 views March 14 is International Day of Mathematics – UNESCO declared it so at its 40th General Conference in 2019 – and also Pi Day (that’s the constant p, not pie) in the US (where March 14 is written in month/day format – 3.14). More