Summers are for primeval pleasures; winters are for festive magic
Finland is too huge to fit everything into one trip, so plan carefully and resist the urge to squeeze everything into one trip. Do you say it's summer? After long, dark, snowbound winters, the Finns welcome the warmer days of summer with a biological eagerness. June to August is an excellent time for hiking and camping in wilderness places such as Urho Kekkonen National Park in Northern Lapland, above the Arctic Circle. It's also a fantastic time to rent a kayak and paddle the Lakeland (there are 188,000 of them), waving to seals as you drift from one beautiful little speck of an island to the next island on the Baltic.
Summer is when Finns retreat from the world and reconnect with nature in remote cottages, some of which are completely off the grid. Days are spent in delightfully primitive ways: searching for fruit, swimming in icy lakes, resting in saunas, and sleeping under a blanket of stars. Juhannus, or midsummer, occurs in late June when families gather for picnics and dance around bonfires.
September in Lapland is peaceful and beautiful, with woods changing gold and crimson and reindeer beginning to rut. When the snow comes in October, the land falls silent, and many attractions and hotels close. In the Arctic north, however, winter offers festive brightness and visits from Santa. With dogsledding, snowmobiling, skiing, and overnight stays in ice hotels, you'll feel like you're in Narnia as the days become shorter. If you're lucky in Lapland, you might see the Northern Lights (statistically October, November, and March are best).