Mistletoe, bequeathed to us from the Druids, has a long association with Christmas revels. Here are a few words about mistletoe from renowned antiquary W. Winwood Reade:

On Christmas Eve it was lately the custom at York to carry mistletoe to the high altar of the Cathedral, and to proclaim a public and universal liberty, pardon and freedom to all sorts of inferior and even wicked people at the gates of the city towards the four quarters of heaven.

Mr. Reade picks up again, coyly, with this account of the custom of kissing under the mistletoe:

But there is one more mistletoe custom which I had almost forgotten. Let us imagine ourselves in the hall of some old-fashioned country mansion. Let it be Christmas night, and at that hour when merriment and wine has flushed every face, and glowed into every heart.

And now I will paint to you a young maiden who embraced in the arms of her lover is whirled round the hall, her eyes sparkling, her white bosom heaving and her little feet scarce seeming to touch the floor. They pause for a moment. An old lady with an arch twinkle in her eye whispers something to her partner, he nods and smiles; she blushes and turns her eyes, pretending not to hear.

They join the dance again, when suddenly he stays her in the centre of the hall. Above their heads droops down a beautiful plant with pale white berries and leaves of a delicate green. He stoops and gives her the kiss-under-the-mistletoe. All laugh and follow his example till the scene vies the revels of the ancient Bacchanals.

--from Mr. Reade'sThe Veil of Isis, or, Mysteries of the Druids, a book most definitely not about Christmas.