Fatto matematico: 821 is a number n for which n, n+2, n+6, and n+8 are all prime.
Trivia: 821 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Data storica: March 31st is the day in 1965 that an Iberia Airlines Convair 440 crashes into the sea on approach to Tangier, killing 47 of 51 occupants.