Adapted from Ronald Hutton's The Stations of the Sun:
Hallowe’en developed from the Celtic feast of Samhain (pronounced 'sow-in'), which marked the end of summer and the beginning of winter. For the Celts, Samhain was the beginning of the year and a time to acknowledge the beginning and the ending of all things. As they looked to nature, they saw the falling of the leaves from the trees, the coming of winter and death. It was a time when they turned to their Gods and Goddesses seeking to understand the turning cycles of life and death. Here, on the threshold of the cold barren winter months, it was also a time of feasting and celebration as the weakest animals were culled to preserve valuable foodstuffs, and provide food to last until the following spring... For the Celts, Samhain was a time when the gates between this world and next were open. It was a time of communion with the spirits of the dead, who, like the wild autumnal winds, were free to roam the earth. At Samhain, the Celts called upon their ancestors, who might bring warnings and guidance to help in the year to come.