general_fact: 47991 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
math_fact: 47991 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
trivia: 47991 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
historical_date: May 24th is the day in 1595 that Nomenclator of Leiden University Library appears, the first printed catalog of an institutional library.