In Celtic polytheism the word druid denotes the priestly class in ancient Celtic societies, which existed through much of Western Europe north of the Alps and in Britain and Ireland until they were supplanted by Roman government and, later, Christianity. Druidic practices were part of the culture of all the tribal peoples called "Keltoi" and "Galatai" by Greeks and "Celtae" and "Galli" by Romans, which evolved into modern English "Celtic" and "Gaulish". They combined the duties of priest, arbitrator, healer, scholar, and magistrate.
The earliest records of the name Druidae (Δρυΐδαι) is found on the works of Greek writers such as Sotion of Alexandria, who was cited by Diogenes Laertius in the second century BCE.
The Druids were polytheists, but also revered elements of nature, such as the sun, the moon, and the stars, looking to them for "signs and seasons". They also venerated other natural elements, such as the oak, certain groves of trees, tops of hills, streams, lakes and even plants, especially mistletoe and holly. Fire was regarded as a symbol of several divinities and was associated with the sun and cleansing. Their calendar year was governed by the lunar, solar, vegetative and herding cycles. The four main holidays include Imbolc (Imbolg) to denote the first signs of spring, Beltane (Beltain) to recognize the fullness of life after spring, Lughnasadh to celebrate the ripening of first fruits and the many-skilled deity Lugh, and Samhain to recognize the end of harvest and the lowering of the barrier between the world of the living and that of the dead. The timing for these four festivals would have been determined by the presence of a full moon and the seasonal changes in the natural world. Imbolc would thus be celebrated at a full moon roughly halfway between the winter solstice and the vernal equinox, Beltane between the vernal equinox and the summer solstice, Lughnasadh between the summer solstice and the autumnal equinox, and Samhain between the autumnal equinox and the winter solstice. This is contrary to popular "New Age" beliefs about Druidism that celebrate a given holiday according to the Julian calendar, which of course did not exist at the time of the formulation of these holidays. In modern times, Imbolc has been transformed into Groundhog Day, elements of Beltane have been absorbed into Easter, and Samhain has become Halloween (or All Hallows' Eve and All Saint's Day).