Fatto generale: 721 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 721 is the smallest number which can be written as the difference of 2 cubes in 2 ways.
Trivia: 721 is a boring number.
Data storica: December 21st is the day in 1946 that an 8.1 Mw earthquake and subsequent tsunami in Nankaidō, Japan, kill over 1,300 people and destroy over 38,000 homes.