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Pi Day is an unofficial holiday that celebrates the mathematical constant pi (π). It is celebrated on March 14 in countries that follow the month/day date format because the digits in this date (3/14) correspond with the first three digits of π (3.14).

In countries that follow the day/month date format, the holiday comes few months later on July 22 and is called Pi Approximation Day. This is because July 22 or 22/7 is the approximate value of π (22/7 = 3.14).

Pi Day on March 14 also coincides with Albert Einstein’s birthday.

Pi Day was founded by Larry Shaw and celebrated for the first time in 1988 at the San Francisco Exploratorium. Public and museum staff marched around a circular space and ate fruit pies.

Students, mathematicians and math enthusiasts all around the world celebrate Pi Day by participating in pi themed activities and contests that include making and eating pies, converting numbers such as the time or one's age using pi, and reciting the digits of pi.