Fatto generale: 1591 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 1591 is the sum of the first 13 numbers that have digit sum 13.
Trivia: 1591 is an unremarkable number.
Data storica: May 9th is the day in 1969 that Carlos Lamarca leads the first urban guerrilla action against the military dictatorship of Brazil in São Paulo, by robbing two banks.