Fatto matematico: 636 is a number n whose 5^{th} root has a decimal part that begins with the digits of n.
Trivia: 636 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Data storica: September 27th is the day in 1066 that William the Conqueror and his army set sail from the mouth of the Somme River, beginning the Norman Conquest of England.