Fait mathématique: 9639 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Culture générale: 9639 is an unremarkable number.
Date historique: May 22nd is the day in 1915 that Lassen Peak erupts with a powerful force, and is the only mountain other than Mount St. Helens to erupt in the continental US during the 20th century.