Fatto generale: 636 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
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 an unremarkable number.
Data storica: September 27th is the day in 1605 that the armies of Sweden are defeated by the Polish–Lithuanian Commonwealth in the Battle of Kircholm.