On June 9th, 1922, Åland became self-governing. 

Autonomy of Åland, Centenary
Issued by Åland in 2022
Designer: Johanna Finne
Artist: Jonas Wilén
First Day Cover with full souvenir sheet and FD cancel

Åland’s Regional Assembly met for the first time June 9, 1922, after decades of conflict over the status of the islands.  

The World Autonomies site has a lengthy, and referenced page on Åland’s quest for self-governing status:

In the early 20th century, a popular movement for what was described as “reunion with Sweden” was growing on Åland in parallel with Finland’s quest for independence. There was turmoil all around, with the First World War evolving, revolution(s) and then a Bolshevik take-over in Russia, and a little later, following Finland’s declaration of independence in late 1917, also a civil war in Finland. This made prospects for the future very unclear. Among other risks, the Ålanders feared the Finnish nationalist movement in an independent Finland and decided to put forward and claim the right to self-determination. On Åland, secret meetings were held, two petitions (December 1917 and June 1919) and other messages were sent to the Swedish king and state organs, and a self-constituted Ålandic Assembly was formed. The leaders of the Åland movement established bilateral contacts with several countries and also engaged in various ways in the Paris Peace Conference of 1919 to promote their cause (Barros 1968; Bring 2002, 9–11; Stephan 2011, 29).

While Sweden promoted the right of Ålanders themselves to decide their future status, in line with the principle of self-determination, Finland demanded that its sovereignty over the Åland Islands be recognized. In 1920, Finland proposed that Åland should become autonomous, and the Finnish parliament even passed a law on autonomy.[19] This law was however rejected as insufficient by the Ålanders. In the end, and after many and various turns, both domestically as well as internationally, it was agreed that the matter be conveyed to the Council of the League of Nations; both Finland and Sweden agreed to this British proposal. First, a Commission of Jurists established that the matter laid within the jurisdiction of the League of Nations, after which a Commission of Rapporteurs was established, with the mandate to find a solution to the dispute (Stephan 2011, 29; Spiliopoulou Åkermark 2009a; Hannikainen 1997, 57).
​Åland Islands | World Autonomies (world-autonomies.info)