WEST PALM BEACH, Fla. (AP) — Math enthusiasts around the world, from college students to rocket scientists, celebrate Pi Day, which is March 14 or 3/14 — the first three digits of an infinite number with many practical uses. Many people will mark the day with a slice of pie — sweet, savory or even pizza. Simply put, pi is a mathematical constant that expresses the ratio of a circle’s circumference to its diameter.