Mathematical fact: 856 is a member of the Fibonacci-type sequence starting with 1 and 9.
Trivia: 856 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Historical date: May 5th is the day in 1972 that Alitalia Flight 112 crashes into Mount Longa near Palermo, Sicily, killing all 115 aboard, making it the deadliest single-aircraft disaster in Italy.