During winter snow frequently closes down many European airports. Sometimes air traffic gets completely stuck in some countries. Not so in Finland though.
Helsinki Vantaa is one of the best equiped airports in the world when it comes to fighting snow and ice. And believe it or not, the longest time Helsinki Vantaa Airport ever was closed due to heavy snow was for 30 minutes. That was in 2003.
So don't worry about the weather if you are planning to fly anywhere from Helsinki around the winter time. Helsinki Vantaa is prepared.