Fatto matematico: 6840 is the number of ways to place 2 non-attacking kings on a 11×11 chessboard.
Trivia: 6840 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Data storica: September 22nd is the day in 1236 that the Lithuanians and Semigallians defeat the Livonian Brothers of the Sword in the Battle of Saule.