Fatto generale: 21540 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21540 is an uninteresting number.
Trivia: 21540 is an uninteresting number.
Data storica: December 21st is the day in 1844 that the Rochdale Pioneers commence business at their cooperative in Rochdale, England, starting the Cooperative movement.