General fact: 515 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Mathematical fact: 515 is the number of graphs on 6 vertices with no isolated vertices.
Trivia: 515 is a boring number.
Historical date: May 29th is the day in 1790 that Rhode Island becomes the last of the original United States' colonies to ratify the Constitution and is admitted as the 13th U.S. state.