Fatto generale: 21690 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21690 is a boring number.
Trivia: 21690 is an unremarkable number.
Data storica: May 20th is the day in 1896 that the six ton chandelier of the Palais Garnier falls on the crowd below resulting in the death of one and the injury of many others.