Fatto generale: 1606 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Fatto matematico: 1606 is the number of strongly connected digraphs with 4 vertices.
Trivia: 1606 is an unremarkable number.
Data storica: May 24th is the day in 1595 that Nomenclator of Leiden University Library appears, the first printed catalog of an institutional library.