From the book, Samhain of Llewellyn’s Sabbat Essentials series

To the ancient Celts, Samhain marked the most important of four Celtic fire festivals. Located halfway between an equinox and a solstice, it is one of four cross-quarter festivals. Every year on the first frost after the full moon in October, families allowed their hearth fires to burn out. At this time, they brought back herd animals from grazing and completed gathering the harvest.

After the fires died, they gathered with the rest of their tribe to observe the Druid priests induced friction with a wheel and spindle: the wheel, representing the sun, turned from east to west and lit sparks. 

To the ancient celts, Samhain marked the completion of the harvest and called them to put their energy into preparing for the coming winter. It also betokened a day when their ancestors would come to visit, followed across the veil by all sorts of creatures both good and bad that moved freely in the mortal world on Samhain night.