Hecho matemático: 2131 is the number of domino tilings of a 3×12 rectangle.
Trivia: 2131 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fecha histórica: October 31st is the day in 1959 that Lee Harvey Oswald attempts to renounce his American citizenship at the US Embassy in Moscow, USSR.