Fait mathématique: 6699 is a strobogrammatic number.
Culture générale: 6699 is a number for which we're missing a fact (submit one to numbersapi at google mail!).
Date historique: May 4th is the day in 1256 that the Augustinian monastic order is constituted at the Lecceto Monastery when Pope Alexander IV issues a papal bull Licet ecclesiae catholicae.