Fatto matematico: 539 is the number of multigraphs with 5 vertices and 9 edges.
Trivia: 539 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Data storica: June 22nd is the day in 1633 that the Holy Office in Rome forces Galileo Galilei to recant his view that the Sun, not the Earth, is the center of the Universe.