Where does the tradition of kissing under mistletoe come from?

Most believe it originated from Norse Mythology

It was first mentioned in "'Twas the Night Before Christmas"

It started at the turn of the 20th Century when mistletoe became plentiful in the United States

In medieval Romania, when Christmas trees were unavailable, people started using mistletoe instead

Most believe it originated from Norse Mythology.

The Norse Mythology goes like this:

Goddess Frigga, goddess of love, has a son named Balder, god of summer sun.

One day, Balder had a dream of death and told his mother about it. She was greatly alarmed because if Balder dies, being a god of summer sun, all life on earth will die too. So, she went to air, water, fire, earth and all the animals and plants to seek their promise not to harm her son, Balder. Unfortunately, she overlooked the mistletoe.

Mistletoes are hemiparasites. They grow on tree trunks or tree branches.

Loki, god of evil, the ultimate enemy of Balder used the mistletoe as an arrow tip and gave the arrow to Hoder, the blind god of winter, to shoot Balder. Hoder did and struck Balder dead.

With the earth's life forms gradually fading and unsuccessfully trying to revive Balder's body, Goddess Frigga held Balder's body and shed tears for him which revived him back to life. The tears fell on the mistletoe and turned into white berries.

Expressing her joy of having her son back, Goddess Frigga kissed everyone who passed under the tree where the mistletoe grew.

The story ended with a decree that whoever stands under the mistletoe, no harm shall befall them, only a kiss, a token of love.