General fact: 5321 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Mathematical fact: 5321 is an uninteresting number.
Trivia: 5321 is a boring number.
Historical date: July 26th is the day in 1533 that Atahualpa, the 13th and last emperor of the Incas, dies by strangulation at the hands of Francisco Pizarro's Spanish conquistadors.