Mathematical fact: 519 is the number of trees on 15 vertices with diameter 5.
Trivia: 519 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Historical date: June 2nd is the day in 1995 that United States Air Force Captain Scott O'Grady's F-16 is shot down over Bosnia while patrolling the NATO no-fly zone.