Hecho general: 520 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Hecho matemático: 520 is the number of ways to place 2 non-attacking kings on a 6×6 chessboard.
Trivia: 520 is a boring number.
Fecha histórica: June 3rd is the day in 1973 that a Soviet supersonic Tupolev Tu-144 crashes near Goussainville, France, killing 14, the first crash of a supersonic passenger aircraft.