Fatto matematico: 21357 is an unremarkable number.
Trivia: 21357 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Data storica: June 21st is the day in 1877 that the Molly Maguires, ten Irish immigrants convicted of murder, are hanged at the Schuylkill County and Carbon County, Pennsylvania prisons.