Mathematical fact: 721 is the smallest number which can be written as the difference of 2 cubes in 2 ways.
Trivia: 721 is an unremarkable number.
Historical date: 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.