Fatto matematico: 6555 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Trivia: 6555 is an unremarkable number.
Data storica: December 11th is the day in 1602 that a surprise attack by forces under the command of the Duke of Savoy and his brother-in-law, Philip III of Spain, is repelled by the citizens of Geneva.