General fact: 5731 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Mathematical fact: 5731 is a value of n for which n (n+2) is a palindrome.
Trivia: 5731 is an unremarkable number.
Historical date: September 9th is the day in 337 that Constantine II, Constantius II, and Constans I succeed their father Constantine I as co-emperors.