The Ultimate Pi Day was on 14th March 1592 because that calendar date had the largest correspondence to the value of Pi since the Julian calendar was introduced.
William Oughtred, a 16th-century English mathematician, is credited to have been the first to propose the sixteenth letter of the Greek alphabet (π) as the symbol for Pi which is the ratio of the circumference of a circle to its diameter.
FYI, Pi Day falls on 14th March, incidentally the birthday of the great Albert Einstein, and the Pi Approximation Day is on 22nd July (22/7).
