Winter is the longest season in Finland. It usually begins around November in most of the territory, though a little later in the southern and coastal regions. In Lapland, it can start as early as mid-October.
In most territories, the winter season lasts for about 100 days until late March or even early April. In Lapland, it lasts for as long as 200 days till mid-May. During this season the Artic Circle countries experience a phenomenon known as 'polar night' when the sun doesn't rise above the horizon. In the northernmost point of Finland, there is no sun for 51 days.
The air is mainly dry and crisp, so you don't get extreme winter temperatures but still extremely warm clothes are a must for a hike or any other outdoor activity. The coldest days occur mainly at the end of January.
Coastal regions get the coldest period a little later in the beginning of February. Snow covers the whole country and all the lakes and even sea freezes. The deepest snow coverage can be found in the middle of March. All over the country, it varies (from 20 to 30cm of snow in the South, to 60-90 cm in Lapland). The ice covering is the thickest in early April about 50-65cm, while in Lapland it can reach 1 m thick.