General fact: 855 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Mathematical fact: 855 is the smallest number which is the sum of 5 consecutive squares or 2 consecutive cubes.
Trivia: 855 is an unremarkable number.
Historical date: May 4th is the day in 2001 that the Milwaukee Art Museum addition, the first Santiago Calatrava-designed structure in the United States, opens to the public.