Fait mathématique: 9871 is the largest 4-digit prime with different digits.
Culture générale: 9871 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Date historique: January 9th is the day in 1349 that the Jewish population of Basel, Switzerland, believed by the residents to be the cause of the ongoing Black Death, is rounded up and incinerated.