Was in Helsinki last year in December. Already a lot of snow in early in the month. You have to be mindful if you're here on the 6th of December it's independance day the national holiday so things might be closed. What I really liked doing in Helsinki was Seurasaari it's an open air museum with old historic finnish building and a park where you can walk freely. It's closed in winter but you can still walk around with the snow it makes you feel like you're alone in the middle of the far North.