General fact: 91921 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Mathematical fact: 91921 is an uninteresting number.
Trivia: 91921 is a boring number.
Historical date: September 2nd is the day in 1807 that the Royal Navy bombards Copenhagen with fire bombs and phosphorus rockets to prevent Denmark from surrendering its fleet to Napoleon.