If you planning a visit to Finland make it between May-Sept. The Midnight Sun occurs in Lapland from mid-June to mid-July. Don't think of visiting there during Winter as it is very short, dark days but amazing winter landscapes.
A reasonable time to spend should be 2 weeks to see Helsinki-Turku region, the Lake area, and Lapland or the Aland islands.
New Year celebrations are especially lively in Helsinki. May Day is an excellent excuse to get wrecked with the locals. June is best with Midsummer's Day at Juhannus, the country's biggest event with parties nationwide. Savonlinna Opera Festival, held in a medieval castle set in a lovely town and Ruisrock festival in pretty Turku.