Today is the summer solstice or midsummer, though most Christian churches have always officially celebrated it on St John's Day, the 24th of June, or rather, a day before, on St John's Eve. Midsummer is still a very important holiday in Sweden and is observed also in Germany. It's rooted in pre-Christian European traditions and is a part of our cultural heritage. It's usually celebrated by bathing in a river which has to do with heathen purification rituals but also with St John baptising people, and lighting a bonfire. In some countries, people decorate their houses with greenery.
It was also time to gather medicinal herbs and to celebrate fertility and pray for a good harvest. You can read more about it over here.