Hecho general: 21573 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Hecho matemático: 21573 is a boring number.
Trivia: 21573 is an unremarkable number.
Fecha histórica: January 23rd is the day in 1973 that a volcanic eruption devastates Heimaey in the Vestmannaeyjar chain of islands off the south coast of Iceland.