Fatto generale: 21720 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 21720 is a boring number.
Trivia: 21720 is an unremarkable number.
Data storica: June 19th is the day in 1987 that Basque separatist group ETA commits one of its most violent attacks, in which a bomb is set off in a supermarket, Hipercor, killing 21 and injuring 45.