Helsinki Travel Guide - Weather
In general, however, summertime is noted for long, mild and sunny days, with daylight in midsummer lasting almost 20 hours. In contrast, winters are predictably bitterly cold, with plenty of snow. The temperature typically stays well below the freezing point during the winter, and it’s cold enough that the sea freezes solid enough for the locals to walk across it.
Winter dœesn’t really end until April, when the temperatures start to rise a bit and the days start to lighten as well. Spring arrives finally in May and is followed by a rather short summer. There’s a local joke that summer in Helsinki lasts ‘from Tuesday through Thursday.’