General fact: 528 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Mathematical fact: 528 is an uninteresting number.
Trivia: 528 is an uninteresting number.
Historical date: June 11th is the day in 1903 that Group of Serbian officers stormed royal palace and assassinated King Alexander Obrenović and his wife queen Draga.