Mathematical fact: 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!).
Historical date: March 31st is the day in 1995 that Selena, an American singer, was murdered by her friend and employee of her boutiques Yolanda Saldívar who was embezzling money from the establishments.