Boo.

Halloween means more than just a day for costumes and candy corn. It's the halfway point between the autumnal equinox and winter solstice, the last of four "cross-quarter" days on the solar calendar.

"Our recognition of the cross-quarter days in the English-speaking world comes primarily from the Celts who lived in Britain in pre-Christian times," said astronomer Richard Pogge at Ohio State University.

Unlike us, Celts, as well as traditional Japanese Shinto societies, considered equinoxes and solstices the middle of a season. They chose cross-quarter dates as a time to ring in the beginning of each season.