Fatto generale: 1559 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 1559 is the smallest prime p with 16 consecutive quadratic residues mod p.
Trivia: 1559 is an unremarkable number.
Data storica: April 7th is the day in 1798 that the Mississippi Territory is organized from disputed territory claimed by both the United States and Spain.