The voting day is 13th of June 2021. If you are unsure who to vote for, there are multiple voting advice applications (vaalikone) that can help you to pick a desired candidate:
Etelä-Suomen Sanomat: https://www.ess.fi/vaalikone/
Early voting in Finland is open 26.5.-8.6. For Finnish citizens living abroad, voting is open 2nd though 5th of June.
Additional information on election policies the municipal elections can be found on the InfoFinland website: Elections and voting in Finland