At Halloween and Samhain, it's said that the veil is thin, between worlds. The Otherworld in Celtic myth is a land where nobody dies or gets old, there's plenty of honey, fruit and wine and treasures of all kinds. It was known as Tir na n-Og, the Land of Youth. At Samhain, the divine race of the other world, the Tuatha De Danann, or Dannanns for short, would go walkabout and freely roam! They came out of the Otherword through the sidhe (pronounced shee) or fairy mounds, to make mischief.
From this sense of collapsed boundaries between worlds came warnings to stay home, and rituals of scaring away spirits -- or playing with them by wearing masks.
The Druids were the shamanic elders of the Celts, not unlike the wizard Gandalf in Lord of the Rings. Rather than being an intermediary for a sky-God, they had intimate knowledge of the terrestrial, ethereal, elemental and the primordial. Some of their wisdom has been lost, because they passed it on through an oral tradition.(2) And yet, what's been lost, can be remembered and reclaimed, as many are doing now.
An established group of reclaimers is The Order of Bards, Ovates and Druids, or OBOD, as it's known, based in Sussex, England. OBOD is an open order that aims to pass on the essential teachings of the indigenous spiritual tradition of the Druids. They offer audio lessons, since druidry was mainly an oral tradition.
(2)Pagan Origins of Halloween, by Rowan Moonstone (free ebook) at Darkbooks.org.