To have a special day for the Sweden-Finns on February 24 was decided only two years ago. The day was chosen because it was the birthday of Carl-Axel Gottlund (1796-1875). Gottlund was born in today's Finland, which was then part of Sweden. He wrote diaries from his journeys among towns and villages in Värmland and other regions in west and central Sweden where many Finnish speaking people lived in minority. He also provided books in Finnish for the locals in order to help keeping the language alive.
Finland.se quotes the chairman of the Sweden-Finn delegation Markuu Peura from an interview in Sisu radio 1.4.2010: (Stockholm News’s translation into English)
“The Sweden-Finns need their own day: a day that not only symbolize ‘the Finnishness’ but a day when one can celebrate everything with a Sweden-Finnish connection. Carl Axel Gottlund, who defended the position of the Finnish language in Sweden already 200 years ago, became a good symbol for this day."
Footnote: The Swedish term Sverige-Finländare (Sweden-Finns) normally refers to people living in Sweden who have heritage from Finland. Their heritage might be either from Finnish speaking or from Swedish speaking people in Finland.
Sweden and Finland formed one and the same country from mid-14th century to 1809 when Finland came under Russian domination.