Druids formed the professional class in ancient Celtic society. They performed the functions of modern day priests, teachers, poets, ambassadors, astronomers, genealogists, philosophers, musicians, theologians, scientists, & judges. Druids led public rituals often held within fenced groves of sacred trees.
The word "Druidae" is of Celtic origin. The Roman writer Pliny the Elder (Gaius Plinius Secundus, 23/24-79 A.D.) believed it to be a cognate with the Greek work "drus," meaning "an oak." "Dru-wid" combines the word roots "oak" & "knowledge" ("wid" means "to know" or "to see" - as in the Sanskrit "vid"). The oak (together with the rowan & hazel) was an important sacred tree to the Druids. In the Celtic social system, Druid was a title given to learned men & women possessing "oak knowledge" (or "oak wisdom").
Some scholars have argued that Druids originally belonged to a pre-Celtic ('non-Aryan') population in Britain & Ireland (from where they spread to Gaul), noting that there is no trace of Druidism among Celts elsewhere - in Cisalpine Italy, Spain, or Galatia (modern Turkey). Others, however, believe that Druids were an indigenous Celtic intelligentsia to be found among all Celtic peoples, but were known by other names.
The Winter Solstice is the time of the death of the old sun & the birth of the dark-half of the year. The Winter Solstice was called "Alban Arthuan," Welch for "Light of Winter" by the Druids. This was a time of dread for the ancient peoples, as they saw the days getting shorter & shorter. A great ritual was needed to revert the course of the sun.
This time for the ritual may have been calculated by the great circles of stone & burial grounds which are aligned to the Winter Solstice, such as Stonehenge in England & Newgrange in County Meath, Ireland. John Aubrey, writing in the 17C first thought it a "probability" that stone circles, such as Stonehenge, "were Temples of the Druids" titling his text on stone circles the "Templa Druidum." This idea was picked up by William Stukeley, in the early 18C, who subtitled his 1st book, Stonehenge, published in 1740, "a Temple Restored to the British Druids, and his 2nd publication on Avebury, published in 1743, "a Temple of the British Druids." Although later, in the 19C, Sir John Lubbock (1834-1913) dated Stonehenge to a period much earlier than the time of the Druids (that is, to about 3000 B.C., whereas the Druids don't appear in the historical record until 1800 years later), nonetheless the view was maintained by some, that Druids were pre-Celtic inhabitants of Britain & that the religious beliefs & practices for which Stonehenge was built are ancestral to those of the laterday Celtic Druids. And the speculation continues.
Sure enough, the next day after the great Druid Winter Solstice celebration, the Sun began to move higher into the sky, showing that it had been reborn. For the Druids, the Winter Solstice is the end of month of the Elder Tree & the start of the month of the Birch. This is the time of the Serpent Days or transformation. The Elder & Birch stand at the entrance to Annwn or Celtic underworld where all life was formed. As in several other Druid myths, they guard the entrance to the underworld. At this time, the Sun God journeys through the underworld to learn the secrets of death & life and to bring out those souls to be reincarnated.
Mistletoe has a compelling Druid history. According to ancient Druid tradition, Mistletoe was the most sacred of all plants. Mistletoe was used by Druid priests in a ceremony which was held 5 days after the New Moon following Winter Solstice. The Druid priests would cut Mistletoe from a holy Oak tree with a golden sickle. The branches had to be caught before they touched the ground. The priest then divided the branches into sprigs & dispersed them to the people, who hung them over doorways as protection. Druids believed Mistletoe had miraculous properties that could cure illnesses, antidote poisons, ensure fertility, & protect against evil witchcraft. It was also a sign of peace & goodwill. When warring tribes came across Mistletoe, a temporary truce would be observed until the next day.
Tradition relates that on the Winter Solstice, Druids would gather by the oldest mistletoe-clad oak. The Chief Druid would make his way to the mistletoe to be cut whilst below, other Druids would hold open a sheet to catch it, making sure none of it touched the ground. With his golden sickle the Chief Druid would remove the mistletoe to be caught below. It is said that the early Christian church banned the use of mistletoe because of its association with Druids.