Editing 1920 (section)

===October===
{{Main|October 1920}}
* October &ndash; English writer [[Agatha Christie]]'s first novel, ''[[The Mysterious Affair at Styles]]'', appears in the U.S., introducing her long-running Belgian detective character [[Hercule Poirot]] in the setting of an [[English country house]]. (The book is published in the U.K. in 1921.)
* [[October 3]] &ndash; The [[Prix de l'Arc de Triomphe]] horse race first runs in Paris.
* [[October 4]] &ndash; The [[Mannerheim League for Child Welfare]], a Finnish [[non-governmental organization]], is founded on the initiative of [[Sophie Mannerheim]].<ref>{{cite web|url=https://www.hs.fi/kaupunki/helsinki/art-2000006380719.html|title=Sophie Mannerheim käveli Kalliossa ja järkyttyi: Sitten hän keksi ovelan juonen, jolla koko Suomi saatiin syytämään apua äärimmäisessä köyhyydessä eläville lapsille|first=Marjaana|last=Varmavuori|work=[[Helsingin Sanomat]]|date=January 22, 2020|access-date=September 15, 2021|language=fi}}</ref>
* [[October 9]] &ndash; [[Polish–Lithuanian War]]: Polish troops take [[Vilnius]].
* [[October 10]] &ndash; [[Carinthian Plebiscite]]: A large part of [[Carinthia (province)|Carinthia Province]] votes to become part of Austria, rather than [[Kingdom of Yugoslavia|Yugoslavia]].
* [[October 14]] &ndash; A peace treaty between the Soviet and the Finnish governments is concluded at [[Tartu]].
* [[October 16]] &ndash; [[Polish–Soviet War]]: After the Polish army captures [[Ternopil|Tarnopol]], [[Dubno]], [[Minsk]] and Dryssa, the ceasefire is enforced.
* [[October 18]] &ndash; Thousands of unemployed demonstrate in London; 50 are injured.
* [[October 26]] &ndash; [[Álvaro Obregón]] is announced as the elected [[president of Mexico]].
* [[October 27]]
**The [[League of Nations]] moves its headquarters to [[Geneva]], Switzerland.
**Baron [[Gerhard Louis De Geer|Louis De Geer the Younger]] becomes the new prime minister of Sweden.
*[[October 30]] &ndash; The [[Communist Party of Australia]] is founded in Sydney.
*[[October 31]] &ndash; Dr. [[Frederick Banting]] of Ontario first records his insight on how to isolate [[insulin]] for the treatment of [[diabetes]]; the first successful human trial of insulin will occur 15 months later.