Tomorrow we celebrate Halloween. Children around the country will be out begging for food and threatening dire consequences if they don’t get it. It seems a strange way to celebrate the evening before a solemn Christian holy day, but there it is.

For Catholics, November 1 is All Saint’s Day, which was originally a day to celebrate Christian martyrs. The evening before All Saint’s Day is Halloween or, more properly, the Eve of All Hallows. On that day, we venerate, or “hallow,” the spirits of all the dead.

If our Halloween practices seem odd given their holy origins, it is because they predate the existence of Christianity by perhaps a thousand years. Early Christians were master propagandists. They were not averse to adopting some of the old religious practices of a culture when they were moving into a new territory. If you’ve ever wondered why you bring a tree indoors at Christmas or distribute eggs on Easter day, that’s the reason.

The objective was to absorb deeply rooted religious practices and rid them of their pagan tendencies. Such was the case with Halloween, but that odd connection will require an astronomical digression.

We base our division of the seasons on the solstices and equinoxes. The summer and winter solstices are the days when the sun spends the most time and the least time above the horizon respectively. They thus mark the longest and shortest days of the year. The summer solstice happens on or near June 21, the winter solstice on or around December 21. We traditionally mark the beginnings of summer and winter on those dates. The practice is strange, given the fact that on June 21 and December 21, the seasons seem well underway.

The equinoxes occur half way between the solstices. The spring equinox occurs on or near March 21, the autumnal equinox on or near September 21. “Equinox” means “equal night,” and the spring and fall equinoxes have almost exactly equal amounts of light and darkness. On those days, we celebrate the beginnings of autumn and spring.

“Cross-quarter days” fall midway between the solstices and equinoxes.

We use solstices and equinoxes to mark the beginnings of our four seasons. However, some cultures divided the year into just two seasons. One of those was the Celtic culture, a loose collection of different tribes spread throughout Europe, Britain, and Ireland.

The Celts recognized two seasons, the light and the dark. They chose cross-quarter days as the beginnings of the seasons.

On or near October 31, a cross-quarter day, they observed the transition from the light part of the year to the dark with a festival called Samhain (pronounced SOW-in), which means “summer’s end.” Samhain, which is roughly equivalent to our New Year’s Eve, marked the final harvest of the year and the beginning of winter.

Samhain thus represented one of two seasonal “seams,” cracks in space and time when the supernatural world could slip into our world.

The Celts looked to the sky to mark the date. Around midnight, look high in the south for the constellation, Taurus, the Bull. A tiny cluster of six stars called the Pleiades forms its shoulder. The Celts considered the Pleiades a portal between the two worlds. When this group of stars was high in the sky at midnight and the moon was full, it was time to celebrate Samhain.

It was a fearful time. On Samhain, the spirits of the dead flooded through the Pleiades to create havoc and damage crops. People would wear disguises to try to hide themselves from the dead. They left their doors open so that the spirits of their dead relatives could visit them.

The dead were famished by their long burial. Apples were buried along roadsides to provide food for spirits without living relatives. People went from door to door asking for contributions of food from their neighbors.

The Celtic priests, called Druids, built large sacred bonfires. People gathered to burn crops and sacrifice animals to the Celtic gods.

Many of the Samhain practices survive in our celebration of Halloween. It took several more centuries for the holiday to get its current name as a result of actions taken first by the Romans and then the Roman Catholic Church over the centuries.

By 43 AD the Roman Empire had conquered most of the Celtic territory. The Romans have the reputation for conquest, but their greatest skill was incorporating aspects of non-Roman culture into their own to encourage assimilation.

Two Roman festivals come to mind. The first was Feralia, a day late in October where Romans marked the passing of the dead. The second was Pomona, the Roman goddess of fruits and trees. The symbol for Pomona is an apple. We still bob for apples on Halloween to this day.

By the seventh century CE, Christianity was firmly in control of the Roman world, but some of the old pagan practices held on. Thus, on May 13, 609, Pope Boniface IV created the Catholic feast of All Martyrs Day at the Pantheon in Rome. Catholicism now had its festival of the dead, at least some of them anyway.

By the ninth century, the influence of Christianity had spread into Celtic lands, where it gradually blended again with the older Celtic rites, which could not be exterminated entirely. In 835, Pope Gregory III expanded All Martyrs Day to include all the saints, martyred and unmartyred, and moved the observance from May 13 to November 1 to replace Samhain.

The Church still needed its own festival of the dead to replace the old Celtic festival. In 1000 CE, All Souls’ Day was born. Significantly, it occurs the day after — and not the day before — All Saint’s Day.

It was, of course, another vain attempt. Our dread-filled love of the night is built into our genetic code. A thousand years later, apples are still bobbed, and tiny ghosts and goblins still pleasantly haunt our neighborhoods.