Finland is two different places in winter and summer. Summer is fairly reliably dry and hot, although by August things can already begin to get chilly. Of course, winters are cold, but the cold is dry. In most parts snow first falls in October and clears by the end of March, but in Lapland snow can fall as early as September and stay until late May.
When to go
The tourist season in southern Finland and the Lakeland is from early June to late August. This is when all attractions and summer accommodation options are open, steamboats and ferries ply the lakes and rivers and festivals are in full swing. Things are at their busiest during Finnish holidays, typically from the summer high season in late June until the end of July. This is the time of long, light nights, when Finland doesn't seem to sleep.
The tourist season in northern Finland, including Lapland, is different. Mosquitoes can be unbearably annoying in July, but September is delightful with its ruska (autumn) colours. October and February/March are good times to visit Lapland to view the aurora borealis (northern lights) and enjoy winter activities such as skiing and dog-sledding. The Christmas holiday period is also prime time in Lapland - after all, this is the 'official' home of Santa Claus. Helsinki is popular year-round.
You can find everything about Finland's weather on the Finnish Meteorological Institute (www.fmi.fi) website.