Fatto matematico: 1661 is a centered dodecahedral number.
Trivia: 1661 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Data storica: July 18th is the day in 1914 that the U.S. Congress forms the Aviation Section, U.S. Signal Corps, giving definite status to aircraft within the U.S. Army for the first time.