"Before 1316, the summer solstice was called Ukon juhla, after the Finnish god Ukko. In e.g. Karelian tradition, many bonfires were burned side by side, the biggest of which was called Ukko-kokko (the "bonfire of Ukko"). After the celebrations were Christianized, the holiday is known as juhannus after John the Baptist (Finnish: Johannes Kastaja).
Since 1955, the holiday is always on a Saturday (between June 20th and June 26th). Earlier it was always on June 24th.
In the Finnish midsummer celebration, bonfires (Finnish kokko)are very common and are burnt at lakesides and by the sea. Often two young birch trees (koivu) are placed on either side of the front door to welcome visitors. In Midsummer night the sauna is typically heated and family and friends are invited to bathe and to grill.
In folk magic, midsummer was a very potent night and the time for many small rituals, mostly for young maidens seeking suitors and fertility. Will o wisps were believed to be seen at midsummer night, particularly to finders of the mythical "fernin bloom" and possessors of the "fern seed", marking a treasure. An important feature of the midsummer in Finland is the white night and the midnight sun. Because of Finland's location spanning around the Arctic Circle the nights near the midsummer day are short or non-existent. This gives a great contrast to the darkness of the winter time.
Many music festivals of all sizes are organized on the Midsummer weekend. It's also common to start summer holidays on Midsummer day. For many families the Midsummer is the time when they move to the countryside to their summer cottage by the lake. Often Finns spend the whole of July at the summer cottages. Midsummerday is also the Day of the Finnish Flag. The flag is hoisted at 6 pm on Midsummer eve and flown all night till 9 pm the following evening." from Wikipedia