Fatto matematico: 1591 is the sum of the first 13 numbers that have digit sum 13.
Trivia: 1591 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Data storica: May 9th is the day in 1671 that Thomas Blood, disguised as a clergyman, attempts to steal England's Crown Jewels from the Tower of London.