General fact: 821 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Mathematical fact: 821 is a number n for which n, n+2, n+6, and n+8 are all prime.
Trivia: 821 is a boring number.
Historical date: March 31st is the day in 2004 that Iraq War in Anbar Province - In Fallujah, Iraq, 4 American private military contractors working for Blackwater USA, are killed after being ambushed.