Hecho general: 21991 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Hecho matemático: 21991 is an uninteresting number.
Trivia: 21991 is an unremarkable number.
Fecha histórica: March 16th is the day in 1950 that Communist Czechoslovakia's ministry of foreign affairs asked nuncios of Vatican to leave the country.