Fatto generale: 21057 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21057 is an unremarkable number.
Trivia: 21057 is an uninteresting number.
Data storica: August 25th is the day in 1939 that the United Kingdom and Poland form a military alliance in which the UK promises to defend Poland in case of invasion by a foreign power.