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.
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.