Fatto matematico: 721 is the smallest number which can be written as the difference of 2 cubes in 2 ways.
Trivia: 721 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Data storica: December 21st is the day in 1844 that the Rochdale Pioneers commence business at their cooperative in Rochdale, England, starting the Cooperative movement.