A few interesting tidbits from Space Weather:

Halloween is a date of astronomical interest: it’s a cross-quarter date, midway between an equinox and a solstice. There are four cross-quarter dates throughout the year, and each is a minor holiday. One is Groundhog Day (Feb. 2nd), another is May Day (May 1st), the third is Lammas Day (Aug. 1st), and the fourth is Halloween (Oct. 31st).

Says John Mosley of the Griffith Observatory: “The Celts of the British Isles used the cross-quarter days to mark the beginnings of the seasons, and winter began with Halloween. Halloween marked the transition between summer and winter, light and dark – and life and death. On that one transitional night those who had died during the previous year returned for a final visit to their former homes. People set out food out for them and lit fires to aid them on their journey — but remained on guard for mischief the spirits might do on the one night when the dead returned to the land of the living.”