Mistletoe has been closely associated with religious rituals since the druids used it in the first century AD, believing that it promoted fertility. That notion spread to Norse mythology, where mistletoe figured prominently in a story about Frigga, the goddess of love. Over the centuries, mistletoe was so associated with pagan beliefs that Christians banned its use in their religious practices.
Ironically enough, though, by 1820, kissing under it had become a popular enough Christmas tradition that famed author Washington Irving wrote about it. But back then, each time there was a kiss under the mistletoe, one its berries was plucked and when all the berries were gone, standing under it no longer required a kiss.
It's probably a good thing we no longer include that part of the tradition--mistletoe berries are poisonous.