Fatto generale: 421 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 421 is the number of commutative monoids of order 6.
Trivia: 421 is an unremarkable number.
Data storica: February 24th is the day in 1996 that the last occurrence of February 24 as a leap day in the European Union and for the Roman Catholic Church.