Hecho matemático: 520 is the number of ways to place 2 non-attacking kings on a 6×6 chessboard.
Trivia: 520 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fecha histórica: June 3rd is the day in 1991 that Mount Unzen erupts in Kyūshū, Japan, killing 43 people, all of them either researchers or journalists.