Today Midsummer is celebrated in Sweden, as well as in several other Nordic/Germanic countries. It is an old pagan holiday, and the only one (at least in Sweden) not hijacked by christianity, despite attempts.
Midsummer celebrates the summer solstice, the shortest night of the year. It actually takes place on June 21, but Midsummer is celebrated on the Friday closes to it, in order to give people a three day weekend.
The traditions in Sweden are that you raise a midsummer pole (sometimes called May pole), usually covered in leaves and flowers. You dance around it, for example the popular "frog dance" (YouTube video here), then eat pickled herring (different varieties) with cooked potatoes and sour cream, bread, cheese, beer and akvavit.