http://www.harpold.com/advent/09.html

What a wonderfully seasonal question!

The tradition of kissing underneath the mistletoe began with a Scandinavian goddess called Frigga and her son Balder, the god of the summer sun. Vikings dating back to the eighth century believed that mistletoe had the power to raise humans from the dead, relating to Balder's resurrection.

Balder had a dream that he was going to die. His mother, Frigga, Odin's wife and the goddess of love and beauty, was frantic about his dream and said that if he died, everything on Earth would die. To ensure her son's safety, Frigga went to all of the elements -- air, fire, water and earth, as well as to all of the animals and plants -- and asked them not to kill Balder. Because of his mother's overprotection, Balder was teased and had things thrown at him. It was thought that, because of his mother's power, he was immune to harm.

Balder's only enemy, Loki, found a loophole in Frigga's request for her son's safety -- mistletoe. Mistletoe grows on the tree it attaches itself to, and therefore has no roots of its own and could not be affected by Frigga's request. Loki made a poisoned dart with mistletoe, and tricked the blind brother of Balder, Hoder, into shooting the arrow that killed Balder.

For three days, all the elements tried their hardest to bring Balder back to life, but failed. Finally, the tears that Frigga cried for her dead son changed the red mistletoe berries to white, raising Balder from the dead. Frigga then reversed mistletoe's bad reputation, and kissed everyone who walked underneath it out of gratitude for getting her son back.

MISTLETOE ETTIQUETTE

Correct mistletoe etiquette is for the man to remove one berry when he kisses a woman. When all the berries are gone, there is no more kissing underneath that plant. It is believed that an unmarried woman not kissed under the mistletoe will remain single for another year.