An Insider’s Guide to Celebrating Pi (π) Day (from Exploratorium edu)Every year on March 14—Albert Einstein’s birthday—number lovers can raise a piece of pie to Pi (π) Day, an international celebration of one of math’s most famous constants: the irrational, never-ending number π, pronounced pi (3.14159 . . .). Exploratorium physicist Larry Shaw invented the playful celebration in 1988, and Congress declared it an official U.S. national holiday in 2009. The San Francisco museum continues its annual festivities with an all-ages Pi Day celebration on March 14, 2020.