Fatto matematico: 891 is the number of unlabeled distributive lattices with 15 elements.
Trivia: 891 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Data storica: June 9th is the day in 1885 that a peace treaty is signed to end the Sino-French War, with China eventually giving up Tonkin and Annam - most of present-day Vietnam - to France.