Fait mathématique: 1539 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Culture générale: 1539 is a boring number.
Date historique: March 18th is the day in 1965 that Cosmonaut Aleksei Leonov, leaving his spacecraft Voskhod 2 for 12 minutes, becomes the first person to walk in space.