Seen as the beginning of the cold season. It is balanced by the corresponding seasonal day of celebration called Beltain which signals the start of summer, 6 months later. Both of these are fire festivals. The ancient Celts probably held them exactly mid-way between an equinox (when day and night were equal) and the following solstice (when the nighttime was shortest or longest).
A time when the veil between this world and the next was at its thinnest. The Celts believed that upon death, everyone went to a beautiful place free of hunger, pain and disease.