Hecho general: 521 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Hecho matemático: 521 is the 13^{th} Lucas number.
Trivia: 521 is an uninteresting number.
Fecha histórica: June 4th is the day in 1979 that Flight Lieutenant Jerry Rawlings takes power in Ghana after a military coup in which General Fred Akuffo is overthrown.