Many cultures, old and new, have holidays or celebration days on similar dates on the calendar. I find it pleasing that they are often tied to the events in the natural world, thus demonstrating that history, as well as technology, has a place in the world. Ancient Celtic cultures celebrated Samhain which meant the end of the summer. Depending upon where you are in the world, you might agree or disagree with that but in Europe and even North America, there has usually not been a night of frost by October 31st.
Harvest is over and, if successful, it would be a time of celebration in an era when people had to be self-sufficient and plan ahead for their needs over the upcoming winter . . . or starve. The Celts also believed that this was the time when the supernatural world and the physical world were in closest contact. The giving of treats originated in the custom of leaving food and drink outside our door to placate the pixies, witches and any random demons that happened to be wandering about. In Celtic times cakes were made for wandering souls, to placate them, a preview of treats handed out to children today. After a period of time, people began dressing up like the demons and witches in a version of whistling past the graveyard.
In the seventh century, the Christian Church chose the day after Halloween--November 1--as the day to honour all Saints who did not have their own day during the year. I remember I was surprised to discover, pursuing some random research, how many Christian saints and martyrs there are and, in consequence, how many days during the year were feast days or holy days in medieval and earlier times. Let's just say early Christians had more holidays than we do today, considerably more.
It's not too much of a stretch to connect Halloween with the other-worldly aspects of Samhain. The celebration of the harvest seems more indicative of Thanksgiving, held in early to mid-October for the colder Canadian climes and closer to the end of November for the balmier U.S. harvest.