General fact: 505 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Mathematical fact: 505 is _{10}C_{5} + _{10}C_{0} + _{10}C_{5}.
Trivia: 505 is an uninteresting number.
Historical date: May 19th is the day in 1934 that Zveno and the Bulgarian Army engineer a coup d'état and install Kimon Georgiev as the new Prime Minister of Bulgaria.