General fact: 21520 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Mathematical fact: 21520 is an uninteresting number.
Trivia: 21520 is an uninteresting number.
Historical date: December 1st is the day in 1913 that the Buenos Aires Subway, the first underground railway system in the southern hemisphere and in Latin America, begins operation.