This traditional decoration that has been used at the time of the winter solstice for centuries. Mistletoe is a plant which lives on trees and bushes, and was particularly associated with the sacred oak tree.
Mistletoe features in Norse mythology in connection with the god Balder. According to one legend, his mother, Frigg, to make him impervious to injury, extracted a promise from all living things not to hurt him. The mistletoe was overlooked, as it was considered too small, so mischievous Loki turned a sprig of mistletoe into a lethal dart.
It played a key part in ancient Celtic wintertime tradition. Druids would ceremonially climb oak trees to cut the mistletoe with a golden sickle. It was not allowed to touch the ground but was caught with a white cloth, and offered to the gods. The ancient Celts called the mistletoe "all heal" (uile-ice) or "sap of the oak" (sùgh-dharaich) as it was believed to be a remedy against poison and infertility. In many European countries is was used for protection against sickness or misfortune.
Because of mistletoe's pagan associations, some say that early Christians disapproved of it and banned it from church. However, many church accounts in the Middle Ages show payments for mistletoe to be used to decorate the church at Christmas. In fact, in York Minster there was a special mistletoe ceremony, where the plant was laid upon the altar and wrongdoers from the city would come to receive a pardon.
Our modern concept of mistletoe connects it with "kissing under the mistletoe," a form of the traditional kissing bough, wherein a sprig of the plant is hung up inside, and any couple passing under are expected to share a kiss. According to the old custom, a berry had to be plucked each time a girl was kissed under it. When all the berries were gone, the kissing had to stop.
The Latin name for the European mistletoe is Viscum Album -- album deriving from the Latin albus "white." In German it is mistel, but it is also known as donnerbesen "thunder broom." The French is gui, vischio is Italian, muérdago is Spanish, jemiola is Polish, imela is Slavic, omela is Russian and misteli is Finnish.