Here's a quick refresher course: Pi is the ratio of a circle's circumference to its diameter. Most of us learned the abbreviated 3.14 number in grade school, but pi can be calculated out infinitely without a discernable repeating pattern. It is both irrational and transcendental.
Einstein starting working at the Institute for Advanced Study in Princeton, N.J., in 1933 and remained associated with it until his death in 1955. Princeton now celebrates Einstein as part of an elaborate "Pi Day weekend" featuring walking tours, pie judging, pie throwing, a pizza competition, and an Einstein lookalike contest, among many other activities.