In Finland you will visibly notice all four seasons, the longest of which seems to be winter but the light and warm days of summer soon make up for the coldness of the darker season. You may have heard stories about the depressing darkness during the winter. Luckily, even during the darkest months southern Finland receives natural light from around 09:00 - 15:30 while in the north of Finland a couple of hours of light is more likely the case. In the summertime, this will be the other way around, when our days are much longer, and during midsummer, the sun does not set at all!
The temperature in Helsinki reaches up to the high twenties (degrees Celsius) in the summer months, and about +10 to +15 during spring.
As winter approaches, the temperature starts to fall below 10 degrees during September and October. By November the snow often begins to fall - usually not melting away until April. The coldest winter month is February where the average daily temperature can reach -6 to -15 degrees. There may be even be the odd day where the temperature in Helsinki falls below - 20 degrees and sometimes even more. The wind can chill the air even more so be sure to wrap up in warm and wear protective clothes on a winter day.