Since Finland is situated quite north, between latitude parallels 60 and 70, one might expect that the weather here is always cold and share some characteristics with Greenland and Northern Canada. Thart’s not actually the case. The climate in Finland is actually dominated by Golf Stream and the climate is a mix between continental and sea climate.
The Polar Front, which is the boundary between Polar and Sub-tropical air masses is usually situated near Finland. In summer it’s usually north of Finland and in winter somewhere over South Finland. Waves in the Polar Front cause depressions that frequently pass over Finland with moderate winds and rain.
Sometimes Finnish weather adopts the characteristics of continental air for some time (usually Siberian) and that may cause a longer period of hot weather (in summer) or cold weather (in winter) with little to no precipitation.
So nothing can really be said for the weather in Finland but most propably it’s raining/snowing from November to February and at that time it’s also colder, temperatures may vary from approximately +4 degrees to -20 degrees in South and it’s even colder in Northern Finland with almost permanent sub-zero temperatures during winter months.
In the summer, temperatures vary from about 15 to 25 degrees in South and again it’s a bit cooler in the North. Finnish weather very rarely adopts the characteristics of tropical climate meaning that the night is very often remarkably cooler than the day.
One other thing to mention is that the amount of daylight from November to February is very little and the days may be surprisingly short (especially in North). During the summer it’s a completely different story with almost 24h/day light (in North that actually happens).
So I personally would suggest you would come in summer from June to August, but September and March-May are also a good choice. For the seekers of extreme, November-February is also ok – especially in the Northern Finland.