Wikipedia:Selected anniversaries/September 20
- 1498 – A tsunami caused by the Nankai earthquake washed away the building housing the statue of the Great Buddha (pictured) at Kōtoku-in in Kamakura, Japan.
- 1643 – First English Civil War: The First Battle of Newbury was fought in Berkshire; Parliamentarian forces were allowed to pass Royalist troops to retreat the next morning.
- 1920 – Irish War of Independence: British police officers known as Black and Tans went on a rampage in Balbriggan as revenge for the shooting of two officers.
- 1977 – A series of celestial phenomena of disputed nature was observed in the western Soviet Union, Finland and Denmark.
- 2000 – The Real Irish Republican Army carried out a rocket-launcher attack on the MI6 headquarters in London, with no casualties and minimal damage recorded.