Here’s how All Hallow’s Eve (Halloween) came to be celebrated

The observance of All Hallow’s Eve, which over the years has been contracted into Halloween, has its roots in the end of summer celebrations held by people of pre-Christian Europe, especially the Celts of the British Isles.

Because the Celts believed that the world of the living and that of the dead were so close during this festival, allowing spirits of the dead to more easily return to the land of the living, people set out food and drink for family members who had died the previous year. Of course, if friendly ghosts could return under this night’s cover of darkness, so could evil spirits. To keep these malevolent ghosts at bay, skeletons, presumably of dead animals, were placed at the entrance of each house and in many cases turnips were hollowed out, faces carved into the shells to represent skulls and small pieces of wood wrapped in oil soaked cloths were placed inside each one and set alight. In some places, people dressed in animal skins or wore grotesque masks to frighten away unwelcome spirits.

In Scotland, the young men disguised themselves as spirits by dressing in white and blackening their faces, or wearing masks or veils to confuse and ward off these unwanted visitors. This costume and carved turnip practice lasted well into mediaeval times, when Christianity had long replaced the Celtic religion.

With the arrival of the Romans in Britain, a new element was added to the end of the summer events, when the festival honouring Pomona, the Roman goddess of fruit and orchards, took place around the same time. Apples played an important role in this celebration.

Another ancient British festival with an apple theme, now known as Snap Apple Night, was celebrated with bonfires, feasting, dancing and itinerant entertainers at this time of year. One of its popular games was to try and bite an apple suspended on a string without using hands. It seems then, that apples and turnips at Halloween time predate pumpkins by many hundreds of years.

With the arrival of Christianity, early church leaders, realizing they couldn’t simply ban these old customs, wisely chose to give them a Christian interpretation and the day remembering Christian saints was moved from May 13 to the end of October and All Hallow's Eve came into being, with the day following called All Saints’ Day (“hallow” is an old English word for holy one or saint).

In parts of 9th century continental Europe, people went from door to door early in November “souling,” because it was believed departed souls needed help on their journey to Heaven. The “soulers” begged specially made little “soul cakes” from each family visited and in return would promise to pray for family members that had recently died. Eventually the day following All Saints’ became All Souls’ Day.

While the Christian aspect of this holy day (holiday) remained strong in the old world, the other old traditions began to fade after 1500, but not before British settlers arrived in Canada and the U.S.A., bringing the old ideas with them. Where now the Christian aspect has become muted, the earlier pre-Christian customs remain strong.

(I shortened the article. More detail at link)