Celtic civilization extended over much of Europe. Today the Celtic people and their culture survive in Scotland, the Isle of Man, Wales, Ireland, Cornwall (SW England), Brittany (NW France) and Galicia (NW Spain). Some of their traditions survive in Catholicism but our greatest understanding of their original spiritual and religious views can be gleaned from their well-known myths and stories.
The cosmos was divided into three main parts: heaven, earth and the Otherworld. The Otherworld, exists alongside the everyday world; the two realities are separated by a veil, this separation is not an impenetrable barrier. The veil also grows thin at the approach of Samhain on November 1st the most important of the four great feasts in the Celtic calendar.
The night of October
31st is a time of transition, when the boundary between the natural
world and the Otherworld lifted, and its spirit inhabitants came
through quite easily. Humans stayed indoors to avoid them, and in
this we find the precursor of modern Halloween.