This article from The Scotsman is 2 years old  but when talking about customs which ago back millennia 2 years is as nothing.  It is also the best article I can find while our dinner cooks about Samhain, the Celtic festival between the autumn equinox and the winter solstice which we know as Halloween.
Samhain was not just about year's end and the coming of winter. It was also the feast of the dead, the season of the earth’s decay when evil was held to wander the planet. The shield of the female warrior Skathcach was lowered, and the barrier between the two worlds faded. The forces of chaos invaded our globe, and the world of the living joined with the world of the dead.
With so much evil pricking the night, man’s response was clear. Fire! Light! Illuminate the dark and cast out the shadows. Banish the witches and warlocks with ritual and rite. Druids built fires, disguising themselves in order to confuse and baffle the evil spirits. They scooped out turnips and fashioned them into skulls and placed them, lit, around their fire to keep evil at bay.
The bonfires of Guy Fawkes on November 5th are much older than the gunpowder plot of 1605. Read more here.