Helsinki has a proper four-season climate and feels like a different city in each. Summer (June–August) is the peak: long evenings that barely darken, terraces full, Suomenlinna's grass slopes covered in picnickers. Midsummer (Juhannus, late June) is the single biggest event of the year — most of the city empties out to the countryside for the long weekend, but what's left is quietly magical. Autumn turns the parks gold and cold; by late October it's dark by 5pm. Winter is snowy and dark but far from dead — Christmas markets on Senate Square, skating rinks open through February, and the sauna culture at its most enjoyable. Spring (April–May) is the transition: muddy at first, then suddenly light again.
The two worst months are November (dark, wet, grey, no snow yet) and early April (dirty slush). Avoid them if you can.