Friday is Pi Day, a national celebration of the mathematical concept, which is the ratio of a circle's circumference to its diameter and equals 3.14... Schools and museums often plan events to celebrate the concept, which has fascinated humans for centuries, and local eateries offer pie-related deals.

Pi Day itself dates to 1988, when physicist Larry Shaw began celebrations at the Exploratorium science museum in San Francisco. The holiday didn’t really gain national recogniti