Some holidays fall on the same date each year because they commemorate a specific event. For example, Independence Day is always July 4 because that is the date when America's 13 colonies declared their independence. That’s simple enough.

Others drift by up to several weeks from year to year. The most familiar example is Easter, which is set as the first Sunday after the first full moon after the spring equinox. When Easter was first celebrated in the early centuries of Christianity, it was set by the same rule as Passover and for the same reason. People didn't have nicely printed calendars to refer to, and they used the sky, especially the motions of the sun and moon, to set important dates.

Perhaps surprisingly, Halloween is also a date that is determined astronomically.

We divide the year into four seasons, and they begin on or near March 21, June 21, Sept. 21 and Dec. 21. These dates are not arbitrary. They mark the time when the sun is at its highest and lowest in the sky and the midpoints. Astronomers can determine those times to the nearest second. Another way of thinking of them as the dates when the sun rises and sets as far north and south along the horizon as it does, and the midpoints.

Midway between these dates, which mark the four quarters of the year, are “cross-quarter” days, and they fall on or near Feb. 1, May 1, Aug. 1 and Nov. 1. We barely note the quarter days and even less the cross-quarter days, but some earlier cultures — especially the Celts — gave them greater importance.

They still linger as Groundhog Day (Feb. 2), May Day (May 1), Lammas (Aug. 1) and Halloween (the evening before Nov. 1). They were important to the Celts as early as the Neolithic Period (as much as 6,000 years ago) as evidenced by the alignments of some ancient passage tombs.

Our modern-day Halloween traces it roots directly to the Celtic "Samhain." In Ireland and Scotland, Samhain marked the end of the harvest and the time when cattle were brought to winter pastures, but more to the point it marked the first day of winter and the first day of the new year.

This “transition” day was dangerous — it was “outside of time” — it lay between light and dark, life and death, when departed ancestors might return to the land of the living before making their final journey to the afterlife. The Celts thought it best to set out food for them and to extinguish all fires; people who went out on this night wore costumes to confuse and ward off the wandering spirits. At midnight new fires were lit to celebrate the new year.

Since the seventh century, Christians had celebrated a day of the dead called All Soul’s Day or All Hallows Day, at various times of the year. It was a time to honor the saints and pray for those who had died in the past year.

In the ninth century, it was standardized to coincide with Samhain to Christianize the pagan holiday. The same thing happened to the Winter Solstice, which became Christmas. The day before All Hallows Day became All Hallows Eve, or Halloween.

Whatever we call it, the pagan customs (like wearing costumes) still shine through rather brightly all around us on this cross-quarter day. Happy Samhain!