The association of mistletoe with Christmas seems, like the “Christmas tree,” to be one more pagan element that has been absorbed into the Christian holiday season. After having a vision that her son Balder will be killed, Norse goddess Frigg (Freya to the Anglo-Saxons) extracts an oath from all animals and plants that they will not harm him. She neglects to get this promise from the mistletoe for the usual obscure reasons, and (in most versions) Loki, the trickster god, kills Balder with an arrow made from mistletoe. After Balder is resurrected, Frigg (using logic that is just so Norse) made the mistletoe an emblem of love.
Hence: kissing under the mistletoe. But, why at Christmas? Mistletoe is an evergreen, and many evergreens were regarded as a rather hopeful sign at the winter solstice. The evergreen nature of mistletoe is made more dramatic by its habit of growing in the branches of deciduous trees. When the tree loses all its leaves in the winter the clumps of mistletoe (several in a tree or in a grove of trees is not unusual) stand out as bright green living things amid the bare branches.
Viscum album is found throughout Europe and east to Nepal. It finds a place in both northern European and Mediterranean mythologies. James Frazer argued in his masterwork The Golden Bough that the eponymous branch cut by Aeneas as a passport to the Underworld was in fact mistletoe. Again, its persistent green-ness through the year makes it a good candidate for maintaining life in a world of the dead.
More generally speaking mistletoe is associated fertility because of its evergreen nature and likely because of its ability to grow even without soil. Saturnalia, the Roman harvest feast, celebrated from the 17th to the 25th of December, was a knockdown drag-out party with lots of drinking and sex. Some sources off-handedly assure us that the tradition of kissing under mistletoe has its roots in the Saturnalia. Perhaps, but its northern European association with the solstice and its pan European association with fertility may simply have come together at Christmas.
In York Minster the pagan symbol is accepted within the church itself at Christmas, reportedly a singular dispensation. A sprig of mistletoe has been hung on the high altar at York since the Middle Ages. Into the 19th century the priests there performed a ceremony of forgiveness and pardon that referenced the plant as a symbol of peace.
The Christmas tradition of hanging mistletoe in the house in a place where single women were likely to walk under it was certainly in place by the 19th century, as both Washington Irving and Charles Dickens refer to it. In those stuff Victorian times a restraint was added to the tradition: with each kiss the man had to pluck a single white berry from the cluster. When they were gone, no more kissing.
As Irving’s reference shows, the mistletoe tradition was carried over to North America, but the mistletoe did not have to be. There is an eastern North American species called Phoradendron serotinum that grows from Long Island to Missouri and southward. It is a member of the same order (Santalales) as the European species and has the same hemi-parasitic habit. They are referred to as “hemi-parasites” because they do photosynthesize and therefore produce some of their own food, but they draw all their water and nutrients directly from the phloem and xylem of the host tree.
The fruit of both species is sticky and transported by birds. Either the birds eat the fruit and excrete the seed on another tree, or they move it after it sticks to them. The common name “mistletoe” (Old English misiltan) is derived from Germanic words mist and tan for “dung” and “twig,” respectively, evidently referring to its mode of dispersal.
There are many native species of mistletoe in the United States, including several members of the genus Phoradendron. Several of them are named for their parasitization of particular tree species (e.g. P. juniperum) and one southeastern US species (P. leucarpum) is known to regularly parasitize oaks, which is interesting because it is thought that the sacred association between oaks and mistletoe in Europe is due to the rarity of finding Viscus album on a Quercus.
The story of the Druids harvesting mistletoe from an oak with a golden sickle is based a short passage written by Roman naturalist Pliny the Elder in the late first century A.D. Aside from other brief and generally prejudiced comments by Roman authors virtually nothing is known of actual Druidic practice. Most of the neo-Druidic rituals originated during the 19th century Romantic period.
It is Pliny who describes the mistletoe being cut by a priest from an oak in the presence of two bulls the sixth day after a new moon. Much of what Pliny wrote jibes with what is known from other Roman accounts and from archaeological evidence.