Fatto generale: 1931 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 1931 is the smallest number whose 7^{th} power has 23 digits.
Trivia: 1931 is a boring number.
Data storica: April 14th is the day in 69 that Vitellius, commander of the Rhine armies, defeats Emperor Otho in the Battle of Bedriacum and seizes the throne.