Wikipedia:Selected anniversaries/September 20
- 1378 – Papal Schism: Unhappy with Pope Urban VI (pictured), a group of cardinals started a rival papacy with the election of Antipope Clement VII, throwing the Roman Catholic Church into turmoil.
- 1854 – The Crimean War began with a Franco-British victory over Russian forces at the Battle of Alma near the River Alma in Crimea.
- 1870 – The Bersaglieri entered Rome, ending the temporal power of the Pope and completing the unification of Italy.
- 1906 – The ocean liner RMS Mauretania, the largest and fastest ship in the world at the time, was launched in Newcastle, England.
- 1946 – The first Cannes Film Festival opened. The Palme d'Or, then known as the Grand Prix du Festival, was shared by eleven films that year.
- 1973 – Billie Jean King defeated Bobby Riggs in straight sets before 30,492 spectators at the Astrodome in Houston, Texas in an internationally televised tennis match dubbed the "The Battle of the Sexes".
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