Did my own search on the definition of "hallows" and found something interesting:

The Oct. 31 Greater Sabbat, also called November Eve, the Celtic Samhain ("sow-en"); the beginning of the Celtic winter, and of the Celtic year; the beginning of the Witches' Year, when the Veil Between the Worlds grows thin and the spirits of the dead may return to Earth; the Descent of the Goddess to the Underworld; the final Harvest festival