Mathematical fact: 645 is the largest n for which 1+2+3+ ... +n = 1^{2}+2^{2}+3^{2}+ ... +k^{2} for some k.
Trivia: 645 is a boring number.
Historical date: October 6th is the day in 1854 that the Great fire of Newcastle and Gateshead starts shortly after midnight, leading to 53 deaths and hundreds injured.