| 23 September 63 BC | Roman Empire [births and deaths] | Augustus, first emperor of the Roman Empire 27 BC–AD 14, born as Gaius Octavius (Octavian) (–AD 14). |
| 23 September 1122 | Holy Roman Empire, Germany [diplomacy] | The ‘Investiture Contest’ (conflict between the papacy and the Empire over lay investiture of senior churchmen) finally ends with the Concordat at Worms, Germany, between Pope Calixtus II and Emperor Henry V of Germany. Henry agrees to end the practice of lay investiture but in practice is allowed to retain some influence over the election of bishops. |
| 23 September 1230 | León, Castile, Spain [political events] | King Alfonso IX of León dies; he is succeeded by his son, Ferdinand III of Castile, who thus finally unites the two kingdoms. |
| 23 September 1625 | United Netherlands [births and deaths] | Johan de Witt, Dutch statesman, political leader of the United Netherlands (1653–72), who led his country during the First and Second Anglo-Dutch Wars, born in Dordrecht, United Netherlands (–1672). |
| 23 September 1822 | Portugal [law and government] | A Portuguese constitution is decreed, providing for liberty, legal equality, a single chamber which the king may not dissolve until its period of four years has expired, and a constitutional monarchy. |
| 23 September 1846 | Germany [astronomy] | German astronomer Johann Gottfried Galle discovers the planet Neptune on the basis of French astronomer Urbain Le Verrier's calculations of its position. |
| 23 September 1939 | Austria [births and deaths] | Sigmund Freud, Austrian neurologist, founder of psychoanalysis, dies in London, England (83). |
| 23 September 1969 | North Vietnam [administration] | Ton Dac Thang succeeds President Ho Chi Minh of North Vietnam following Ho Chi Minh's death on 3 September. |
| 23 September 1974 | UK [television] | The British Broadcasting Corporation (BBC) launches CEEFAX, a television information system, in Britain. |
| 23 September 1975 | Israel, Egypt [political events] | Israel and Egypt reach an agreement on an Israeli withdrawal from the Egyptian Sinai peninsula, which has been occupied by Israel since 1967. |
| 23 September 1978 | USA [popular music] | The US new wave group Blondie releases the album Parallel Lines. |
| 23 September 1988 | South Korea, Canada [athletics] | At the Seoul Olympic Games in South Korea, the Canadian runner Ben Johnson wins the 100 metres in a world record time of 9.79 seconds; he is then stripped of the title three days later when drug tests reveal traces of an anabolic steroid, stanozol. |
| 23–30 September 2005 | USA [natural disasters] | Only about three weeks after Hurricane Katrina battered the southern states on the Gulf coast of the USA and flooded the city of New Orleans, Hurricane Rita strikes Louisiana again and also Texas. However, Rita is less ferocious and the federal and state authorities, stung by accusations of inaction and complacency in the Katrina disaster, are better prepared, implementing a civil evacuation to minimize casualties. By the end of the month, the death toll from Hurricane Katrina is believed to have reached about 1,200, much less than originally feared. |
