Hecho matemático: 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 an uninteresting number.
Fecha histórica: October 6th is the day in 1600 that Jacopo Peri's Euridice, the earliest surviving opera, receives its première performance in Florence, signifying the beginning of the Baroque Period.