Fatto generale: 21537 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21537 is an uninteresting number.
Trivia: 21537 is a boring number.
Data storica: December 18th is the day in 1917 that the resolution containing the language of the Eighteenth Amendment to enact Prohibition is passed by the United States Congress.