Fatto generale: 539 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 539 is the number of multigraphs with 5 vertices and 9 edges.
Trivia: 539 is an uninteresting number.
Data storica: June 22nd is the day in 1774 that the British pass the Quebec Act, setting out rules of governance for the colony of Quebec in British North America.