What we refer to, today, as Druidism, the religion of the Celts (Britain 1500BC), was in fact, not a religion, but a title. Because the Druids used an oral, not written, system of recording, all knowledge of their existence comes primarily from Roman and Greek observations and records. The Celts had three classes: the Bards, the Ovates and the Druid.
The Bard was the keeper of tradition; of the memory of the tribe; the custodian of the sacredness of the Word. In Ireland, they trained for 12 years learning grammar, hundreds of stories, poems, philosophy, the Ogham tree-alphabet. Some Druidic teachings survived in the Bardic colleges in Wales, Ireland and Scotland which remained active until the 17th century, in medieval manuscripts, and in oral tradition, folklore and ritual.
The Ovate worked with the processes of death and regeneration. They were the native healers of the Celts. They specialized in divination, conversing with the ancestors, and prophesizing the future, travelling beyond the grave to bring peace and counsel to those still living on earth. The Ovate was the doctor, detective, diviner and seer.
The Druid was the professional class in Celtic society, performing the functions of modern day priests, teachers, ambassadors, astronomers, genealogists, philosophers, musicians, theologians, scientists, poets and judges. Druids led all public rituals, which were normally held within groves of sacred trees. In their role as priests, they acted not as mediators between God and man, but as directors of ritual, as shamans guiding and containing the rites. To achieve the full status of Druid, training took up to twenty years.
In Roman writing, there is made mention of female Druids, but not whether they were equal to their male counterparts, or had restricted duties. During the reign of Christian rule, there is no mention of women in relation to the Druids.
The druids are known to have taught metempsychosis, or the transmigration of souls; we are reborn in new bodies, sometimes human, sometimes animal. This is not a punishment, but seen as a natural part of existence. The Celts did not have a philosophy that "this world is evil and must be escaped" Instead, "wine, women, and song" were celebrated at all times, and being reborn was considered a good thing. The Celts were said to be unafraid of death, because of their belief in this rebirth.
Trees were incredibly important to the Celts. The word "Druid" is even thought by some to come from the Gaelic word "DUIR" meaning oak. People speculate this today for two reasons: first, duir and druid look fairly alike, and also, the oak tree is a very important tree to the Celts because it is strong, tall, and very long-lived. Trees as a whole were important to the Celts because they held the three realms in them, or at least connected them. They connected the ground and the sky, and transported water through them. When the three realms came together, it was thought to be very powerful place, and was a preferred place to cast spells, practice divination, and to write poetry. Other trees important to the Celts were the Yew, whose offspring grow from the stump of the parent which to them meant perpetually regenerating life. The Birch, the Rowan, Ash, Alder, Willow, Hawthorn, Holly, Hazel, Apple, Vines, Ivy, Reed, Blackthorn, Elder, Silver Fir, Furze, Heather, and Poplar were also important trees, and were so important that the Celts used them in the Ogham written language. Each tree stood for a different letter.
The Celts did not form a single religious or political unity. They were organized into tribes spread across the British Isles, including Ireland, Scotland and wales. As a result, of the 374 Celtic deities which have been found, over 300 occur only once in the archeological record; they are believed to be local deities. There is some evidence that their main pantheon of Gods and Goddesses might have totaled about 3 dozen - perhaps precisely 33 (a frequently occurring magical number in Celtic literature). Some of the more famous are: Arawn, Brigid, Cernunnos, Cerridwen, Danu, Herne, Lugh, Morgan, Rhiannon and Taranis. Many Celtic deities were worshipped in triune (triple aspect) form. Triple Goddesses were often sisters.
There were two major festivals in the practice of the Celts, when the population came together to offer their prayers to their god. One was in early May, and was called Beltane. It is the joy and relief expressed in early spring that the harsh winter is over and the plants return to life. Beltane, meaning "fire of God" was the return of the sun and longer days. It is still honored in Europe as May Day.
The second festival was in November, called Samhain, and survives as Hallows Eve. It was the renewal of fires of the hearth, in preparation for winter. All fires in the village would be extinguished, and rekindled from the central blessed fire of the Druids. Judgements on criminals were held at this time, and other questions regarding public and private property were answered during the conclave of Druid priests. We can assume that the inter-tribal meeting also served many other social functions, as in marriages, and trade of information and goods.