The early Christian church chose Midsummer Day as the birth date of Saint John the Baptist (June 24th -- it isn't really "mid summer" but that's the day traditionally celebrated as midsummer's day).
Saint John has long been thought of as one of the greatest saints. It was he who told of the coming of Christ. He was given the name John the Baptist because he baptized people -- including Jesus.
Saint John's Day has been celebrated throughout many Christian countries for hundreds of years.
In Puerto Rico, for example, bonfires blaze the entire night of June 23rd. Between midnight and dawn, people swim in the sea in memory of the saint who baptized people in water.
In Quebec, Canada, the Fete de la Saint-Jean-Baptiste is also an important holiday which includes bonfires, fireworks, parades and carnivals.