Wikipedia:Selected anniversaries/September 20
- 451 - Attila, king of the Huns, invaded Gaul, but was defeated by Aetius with the help of Roman Foederati in the Battle of Chalons.
- 1378 - Papal Schism: Unhappy with the behavior of Pope Urban VI, a group of cardinals elected a rival papacy in Antipope Clement VII, throwing the Church into a turmoil.
- 1854 - The Crimean War began with a Franco-British victory over Russian forces at the Battle of Alma.
- 1946 - The first Cannes Film Festival began. Eleven films shared the Palme d'Or that year.
- 1973 - Billie Jean King thrashed Bobby Riggs in straight sets before 30,492 spectators at the Astrodome in Houston, Texas in an internationally televised tennis match dubbed as the "Battle of the Sexes".
Recent days: September 19 – September 18 – September 17