Mistletoe is a staple of Christmas tradition and the habit of kissing beneath it stretches back to the time of Norse legend. Baldur, son of Odin and Frigga began to dream foretellings of his own death. Desperate to save him Frigga went round to every being and begged them to swear to do no harm to her son. All agreed, but for mistletoe, who was far too young to be asked.
Amused by his new found invulnerability the gods would throw objects at Baldur and laugh as they bounced off harmlessly. One day, trickster god, Loki handed the blind Hodr an arrow of mistletoe and guided his throw. The arrow pierced Baldur's breast and killed him. In the more cheerful versions of the story, Frigga was so distraught by her son's death that she wept over his body. When her tears touched it, the arrow began to grow stunning white berries. Frigga placed them on the wound, which healed, resurrecting Baldur (though in other versions this healing does not occur). In gratitude and memory of the plant's actions, Frigga blessed the plant. All who passed beneath would share a kiss.
Originally this tradition meant that guests would kiss the hand of their host upon arriving at their house, nowadays the tradition can refer to any kiss between anyone caught beneath the plant and it is typically only hung at Christmas.