When and where did the tradition of kissing under the mistletoe originate?

Great question, I've never really thought about it but like most christmas traditions it probably has a pagan origin, maybe search pagan traditions rather then Christmas for the answer :)

I found this interesting: http://christmaskissingmistletoe.com/KissingMistletoe.htm

It's a Norse tradition. Baldr, the god of light and beauty, was the son of Odin and Frigg. Baldr and his mother started having dreams about his death, so his mother made every living thing make a vow never to hurt her son. Every living except the mistletoe, that is—she thought the plant was insignificant so she didn't bother. All the gods made a sport of throwing everything they could find at Baldr, which would magically bounce off of him. The trickster, Loki, found out about the mistletoe not having made a vow. He tricked Baldr's blind brother Hod into shooting Baldr with an arrow of mistletoe, which killed him. Subsequently Odin impregnated a giantess whose son was born a day later and slaughtered Hod. Frigg, saddened by Baldr's death, but determined to make the best out of things, made it tradition that two under mistletoe should kiss to honour Baldr's memory.