Mathematical fact: 863 is a value of n so that n(n+6) is a palindrome.
Trivia: 863 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Historical date: May 12th is the day in 1264 that the Battle of Lewes, between King Henry III of England and the rebel Simon de Montfort, 6th Earl of Leicester, begins.