Midsummer is the longest day of the year. It’s mostly celebrated by Europeans. It is especially important in the Scandinavian cultures, for example in Finland it’s the most celebrated holiday after Christmas.
The origins of Midsummer come from the pre-Christian era. Even though it’s mostly celebrated in Finland, it’s also celebrated in Germany, Ireland, parts of Britain (Cornwall especially), France, Italy, Malta, Portugal, Spain, Ukraine, other parts of Europe, and elsewhere - such as Canada, the United States, Puerto Rico, and also in the Southern Hemisphere (mostly in Brazil, Argentina and Australia).
It takes place between June 21st and June 24th, depending from the culture.
In Finland the name for the Midsummer is juhannus. It comes from the times when the fest was christianized and took the name from John the Bapist (Johannes Kastaja). Before getting christianized it was called Ukon juhla, after the traditional Finnish god. Bonfires are really commonly burned by lakesides and a couple of birch trees are placed on both sides of the door to welcome guests into house.
A traditional bonfire in Pielavesi (http://files.myopera.com/MrFukov/albums/90209/DSC06748_2.jpg)