Fatto generale: 1421 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 1421 is a value of n for which s(f(n)) = 2s(n).
Trivia: 1421 is an unremarkable number.
Data storica: November 21st is the day in 1905 that Albert Einstein's paper, Does the Inertia of a Body Depend Upon Its Energy Content?, is published in the journal "Annalen der Physik".