General fact: 521 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Mathematical fact: 521 is the 13^{th} Lucas number.
Trivia: 521 is an uninteresting number.
Historical date: June 4th is the day in 2010 that Falcon 9 Flight 1 was the maiden flight of the SpaceX Falcon 9 rocket, which launched from Cape Canaveral Air Force Station Space Launch Complex 40.