Fatto generale: 15815 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 15815 is a boring number.
Trivia: 15815 is an uninteresting number.
Data storica: April 19th is the day in 1809 that an Austrian corps is defeated by the forces of the Duchy of Warsaw in the Battle of Raszyn, part of the struggles of the Fifth Coalition.