General fact: 668 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Mathematical fact: 668 is the number of legal pawn moves in Chess.
Trivia: 668 is a boring number.
Historical date: October 29th is the day in 312 that Constantine the Great enters Rome after his victory at the Milvian Bridge, stages a grand adventus in the city, and is met with popular jubilation.