Fatto generale: 1666 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 1666 is the sum of the Roman numerals.
Trivia: 1666 is an uninteresting number.
Data storica: July 23rd is the day in 1984 that Vanessa Williams becomes the first Miss America to resign when she surrenders her crown after nude photos of her appeared in Penthouse magazine.