The tradition started when Finns started celebrating it, because it is the longest day in the year. We call it Juhannus according to Saint John. It's part of the tradition to burn a big bonfire, which is called kokko here. The wood for the kokko is gathered the whole tear and in spring birds make their nests in the bonfire, and their nestlings die when the kokko is burnt.
Conservationists oppose to it.
It is part of the midsummer tradition that women roll around in the fields naked, and collect seven different flowers and hope that they could dream about their future husband. We have thousands of lakes here, so most of the population try to go to their Summer cottages and spend Midsummer eve on a lake. It can be a very beautiful sight. The nightless day or white night, or day without end is the shorstest Saturday night for some people here. Many people get married at Midsummer because it is so romantic to have a Midsummer wedding in the countryside.