| 119897 | October 20 | Event | 1883 – Peru and Chile sign the Treaty of Ancón, by which the Tarapacá province is ceded to the latter, bringing an end to Peru's involvement in the War of the Pacific. |
| 119898 | October 20 | Event | 1904 – Chile and Bolivia sign the Treaty of Peace and Friendship, delimiting the border between the two countries. |
| 119899 | October 20 | Event | 1910 – The hull of the RMS Olympic, sister-ship to the ill-fated RMS Titanic, is launched from the Harland and Wolff shipyard in Belfast, Northern Ireland. |
| 119900 | October 20 | Event | 1935 – The Long March, a mammoth retreat undertaken by the armed forces of the Chinese Communist Party a year prior, ends. |
| 119901 | October 20 | Event | 1939 – Pope Pius XII publishes his first major encyclical, entitled Summi Pontificatus. |
| 119902 | October 20 | Event | 1941 – World War II: Thousands of civilians in Kragujevac in German-occupied Serbia are murdered in the Kragujevac massacre. |
| 119903 | October 20 | Event | 1944 – The Soviet Army and Yugoslav Partisans liberate Belgrade, the capital of Yugoslavia |
| 119904 | October 20 | Event | 1944 – Liquefied natural gas leaks from storage tanks in Cleveland and then explodes the explosion and resulting fire level 30 blocks and kill 130. |
| 119905 | October 20 | Event | 1944 – American general Douglas MacArthur fulfills his promise to return to the Philippines when he commands an Allied assault on the islands, reclaiming them from the Japanese during the Second World War. |
| 119906 | October 20 | Event | 1946 – Government of the Democratic Republic of Vietnam decides that October 20 is Vietnam Women's Day. |
| 119907 | October 20 | Event | 1947 – The House Un-American Activities Committee begins its investigation into Communist infiltration of Hollywood, resulting in a blacklist that prevents some from working in the industry for years. |
| 119908 | October 20 | Event | 1947 – The United States of America and the Islamic Republic of Pakistan establish diplomatic relations for the first time. |
| 119909 | October 20 | Event | 1951 – The "Johnny Bright incident" occurs in Stillwater, Oklahoma |
| 119910 | October 20 | Event | 1952 – Governor Evelyn Baring declares a state of emergency in Kenya and begins arresting hundreds of suspected leaders of the Mau Mau Uprising, including Jomo Kenyatta, the future first President of Kenya. |
| 119881 | October 19 | Death | 2015 – Ena Kadić, Bosnian-Austrian model (b. 1989) |
| 119882 | October 19 | Death | 2015 – Fleming Mackell, Canadian ice hockey player and singer (b. 1929) |
| 119883 | October 19 | Death | 2015 – Ali Treki, Libyan politician and diplomat, Libyan Minister of Foreign Affairs (b. 1938) |
| 119884 | October 19 | Holiday and observance | Christian feast day:Aaron (Coptic Orthodox Church of Alexandria)Blessed Jerzy PopiełuszkoFrideswideHenry Martyn (Anglican Communion)Isaac Jogues, Jean de Brébeuf, and CompanionsPaul of the CrossWilliam Carey (Episcopal Church)October 19 (Eastern Orthodox |
| 119885 | October 19 | Holiday and observance | Constitution Day, in honor of the country's independence (self-governing in free association with New Zealand) in 1974. (Niue) |
| 119886 | October 19 | Holiday and observance | Mother Teresa Day (Albania) |
| 119887 | October 20 | Event | 1548 – The city of Nuestra Señora de La Paz (Our Lady of Peace) is founded by Alonso de Mendoza by appointment of the king of Spain and Holy Roman Emperor, Charles V. |
| 119888 | October 20 | Event | 1572 – Relief of Goes: Cristóbal de Mondragón, with 3000 soldiers of the Spanish Tercios, relieves the siege of the city. |
| 119889 | October 20 | Event | 1720 – Caribbean pirate Calico Jack is captured by the Royal Navy. |
| 119890 | October 20 | Event | 1740 – Maria Theresa takes the throne of Austria. France, Prussia, Bavaria and Saxony refuse to honour the Pragmatic Sanction and the War of the Austrian Succession begins. |
| 119891 | October 20 | Event | 1781 – The Patent of Toleration, providing limited freedom of worship, is approved in Habsburg Monarchy. |
| 119892 | October 20 | Event | 1803 – The United States Senate ratifies the Louisiana Purchase. |
| 119893 | October 20 | Event | 1805 – General Mack's army surrenders to Napoleon at Ulm after a few skirmishes. |
| 119894 | October 20 | Event | 1818 – The Convention of 1818 is signed between the United States and the United Kingdom, which settles the Canada–United States border on the 49th parallel for most of its length. |
| 119895 | October 20 | Event | 1827 – In the Battle of Navarino, a combined Turkish and Egyptian fleet is defeated by British, French, and Russian naval forces in the last significant battle fought with wooden sailing ships. |
| 119896 | October 20 | Event | 1873 – Yale, Princeton, Columbia, and Rutgers universities draft the first code of American football rules. |