Transport yourself back to ancient times, and place yourself into the Celtic culture. It’s the end of autumn, and you’re around a bonfire dressed up to ward off the dead. This is the holiday known to the ancient Celts as Samhain (pronounced sow-in).
Originally celebrated on 1.November, Samhain was a festival to mark the end of summer and harvest and the beginning of the long, cold, dark winter. The Celts, who lived 2,000 years ago, believed that the on the eve of Samhain, 31.Oct, the boundary between the human world and world of the dead were blurred. During this time, they believed that ghosts came back to Earth to cause trouble and damage crops.
Also during this time, the Celts believed the veil between the worlds was thin, and it was the best time to do divination and get information from the spirits. This is where many modern rituals come from, such as snap-apple game that is so well loved.
One such archaeological site exists at the Tlachtga or Hill of Ward, which is located near Athboy in Meath. The ringfort on top is a place of legend. According to these legends, druids would gather there to light huge fires as a signal to the beginning of the festivities.
By the 43 A.D. Romans had conquered much of the area that the Celts inhabited. During the time that the Romans and Celts coexisted, they blended their religious festivals. The Celts had Samhain, meanwhile the Romans had Feralia where they remembered the passing of the dead and Pomona, a festival that honoured the Roman goddess of fruit and trees. Pomona symbol was an apple, and this is sometimes thought to be where many other apple games come from, such as bobbing for apples.
On 13.May in 609 A.D., Pope Boniface the 4th established All Martyrs Day. This was to be later expanded upon by Pope Gregory the 3rd to include all the saints as well, and moved to 1.November. All Souls Day, a Catholic holiday celebrating the dead in much the same way as the ancient Celts celebrated Samhain began to be celebrated around this time. It is believed that it was done this way not only to blend with the Celtic culture, but also to replace pagan traditions with those sanctioned by the Church.
Linguistically speaking, this is where we start to get the term Halloween. In Middle English All Saints Day was known as Alholowmesse. This was shortened to All-hallowmas or All-hallows, and eventually Halloween.
To learn more about the traditions surrounding Halloween, such as witches, magic, zombies and vampires, see this lovely compilation on the Archaeology archive.