Fatto generale: 21787 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21787 is an unremarkable number.
Trivia: 21787 is an unremarkable number.
Data storica: August 25th is the day in 1537 that the Honourable Artillery Company, the oldest surviving regiment in the British Army, and the second most senior, is formed.