Halloween – short for All Hallows’ Eve – is an astronomical holiday. Sure, it’s the modern-day descendant from Samhain, a sacred festival of the ancient Celts and Druids in the British Isles. But it’s also a cross-quarter day, which is probably why Samhain occurred when it did. Early people were keen observers of the sky. A cross-quarter day is a day more or less midway between an equinox (when the sun sets due west) and a solstice (when the sun sets at its most northern or southern point on the horizon). Halloween – October 31 – is approximately midway point between the autumn equinox and winter solstice, for us in the Northern Hemisphere.
In other words, in traditional astronomy, there are eight major seasonal subdivisions of every year. They include the March and September equinoxes, the June and December solstices, and the intervening four cross-quarter days.
In modern times, the four cross-quarter days are often called Groundhog Day (February 2), May Day (May 1), Lammas (August 1) and Halloween (October 31).
Halloween is the spookiest of the cross quarter days, possibly because it comes at a time of year when the days are growing shorter. On Halloween, it’s said that the spirits of the dead wander from sunset until midnight. After midnight – on November 1, which we now call All Saints’ Day – the ghosts are said to go back to rest.
The October 31 date for Halloween has been fixed by tradition. The true cross-quarter day falls on November 7, representing a discrepancy of about one week. According to the ancient Celts, a cross-quarter day marks the beginning – not the middle – of a season.
The Pleiades connection. It’s thought that the early forbearer of Halloween – Samhain – happened on the night that the Pleiades star cluster culminated at midnight.
In other words, the Pleiades climbed to its highest point in the sky at midnight on or near the same date as this cross-quarter day. In our day, Halloween is fixed on October 31, though the midnight culmination of the Pleiades cluster now occurs on November 21.
Presuming the supposed connection between Samhain and the midnight culmination of the Pleiades, the two events took place on or near the same date in the 11th century (1001-1100) and 12th century (1101-1200). This was several centuries before the introduction of the Gregorian calendar.
At that time, when the Julian calendar was in use, the cross-quarter day and the midnight culmination of the Pleiades fell – amazingly enough – on or near October 31. But, then, the Julian calendar was about one week out of step with the seasons. Had the Gregorian calendar been in use back then, the date of the cross-quarter day celebration would have been November 7.
But Halloween is now fixed on October 31. Meanwhile, the true cross-quarter day now falls on or near November 7 and the midnight culmination of the Pleiades cluster on or near November 21.
Bottom line: The present date for Halloween – October 31 – marks the approximate midway point between the autumn equinox and the winter solstice. Halloween is one of the year’s four cross-quarter days. It is the modern-day descendant from Samhain, a festival of the ancient Celts and Druids. The Pleiades star cluster also plays a role in this story, because Samhain was said to happen on the night that the Pleiades star cluster culminated – or reached its highest point in the sky – at midnight.
Bruce McClure has served as lead writer for EarthSky's popular Tonight pages since 2004. He's a sundial aficionado, whose love for the heavens has taken him to Lake Titicaca in Bolivia and sailing in the North Atlantic, where he earned his celestial navigation certificate through the School of Ocean Sailing and Navigation. He also writes and hosts public astronomy programs and planetarium programs in and around his home in upstate New York.