Fatto generale: 891 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 891 is the number of unlabeled distributive lattices with 15 elements.
Trivia: 891 is a boring number.
Data storica: June 9th is the day in 1978 that the Church of Jesus Christ of Latter-day Saints opens its priesthood to "all worthy men", ending a 148-year-old policy excluding black men.