Mathematical fact: 8506 is the number of isomers of C_{13}H_{26} without any double bonds.
Trivia: 8506 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Historical date: April 15th is the day in 1632 that Battle of Rain; Swedes under Gustavus Adolphus defeat the Holy Roman Empire during the Thirty Years' War.