Summer Solstice

The celebration of Midsummer's Eve was from ancient times linked to the summer solstice. Some people believed that mid-summer plants, especially Calendula, had miraculous healing powers and they therefore picked them on this night. Bonfires were lit to protect against evil spirits which were believed to roam freely when the sun was turning southwards again. In later years, witches were also thought to be on their way to meetings with other powerful beings.

http://en.wikipedia.org/wiki/Midsummer#History