On September 20
451 The Battle of Châlons took place in North Eastern France. Flavius Aetius's victory over Attila the Hun in a day of combat is considered to be the largest battle in the ancient world.
1066 The Battle of Fulford: King Harald III of Norway (Harald Hardrada) and Tostig Gowinson, his English allay, defeated Edwin of Mercia and Morcar of Northumbria.
1187 Saladin began the Siege of Jerusalem. This act of aggression provoked the Third Crusade.
1378 Cardinal Robert of Geneva, called by some the Butcher of Cesena, was elected as Avignon Pope Clement VII, beginning the Papal schism.
1633 Galileo Galilei was tried before the Congregation for the Doctrine of the Faith for teaching that the Earth orbits the Sun and was found "vehemently suspect of heresy", forced to recant, and spent the rest of his life under house arrest.