Mathematical fact: 639 is a number n whose 5^{th} root has a decimal part that begins with the digits of n.
Trivia: 639 is an unremarkable number.
Historical date: September 30th is the day in 1791 that the first performance of The Magic Flute, the last opera by Mozart to make its debut, took place at Freihaus-Theater auf der Wieden in Vienna, Austria.