Fatto generale: 991 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 991 is an uninteresting number.
Trivia: 991 is a boring number.
Data storica: September 17th is the day in 1631 that Sweden wins a major victory at the Battle of Breitenfeld against the Holy Roman Empire during the Thirty Years War.