How it all began

Halloween started as a Celtic pagan holiday, much like many holidays we now equate with Judeo-Christian feast days. For most holidays, the Christian Church replaced the pagan elements with their own. Well, they were in charge during the Dark Ages, and felt entitled.

So anyway, during the reign of the Roman Empire, Pomona Day was celebrated. Pomona was a woman somehow related to the goddess Venus and she was into gardening. She had many suitors but disregarded them. Why the Romans had a day set aside for such a stick in the mud, probably had something to do with them needing another excuse to get drunk and not do any work, which was in the end why the Roman Empire fell. Every day was a holiday for them and they stopped getting around to working. Well, after you've conquered the entire known world, there's nowhere else to go but down.

Meanwhile, the superstitious followers of Celtic traditions in what is now western Europe were concerned about their harvests in the autumn, and they also believed this was the time of the year when their sun god was captured by Samhain, who ruled the world of the dead. This, they believed, was why things got colder all the sudden, and 1 November was to them the start of winter and of a new year, the old one having been ended because Samhain was being a jerk2.

The evening before their new year, the Celts believed Samhain gathered all the dead together to party and give unprepared living people a hard time. Evil ghosts would take to inhabiting black cats and other animals.

This meant certain death for anyone still alive, so the druids and the leaders of the Celtic tribes came up with a plan. This plan was meant to convince the villagers that all the evil spirits were sufficiently scared away for the next several months until the sun god came back. It was also supposed to be a massive blowout party so people worried about such things as sun gods being captured by princes of darkness would forget their troubles and get exceedingly drunk.

So the druids and leaders of the Celtic tribes would start massive fires as the moon rose in the sky, offer sacrifices to the gods, tell everyone to dress up in ugly costumes that would scare off any evil spirits that happened by, drink a lot, and then the following morning they would give embers of the fires to the people of the village, who in turn went home to light their own fires. In this way, it was believed all the Celtic homes would be kept warm and protected from Samhain and his rowdy deadhead friends.

Eventually the Romans invaded the Celts, and their social cultures were intermingled. The Samhain festival and Pomona Day got mixed up until no one knew what they were celebrating exactly. Still, the fires were warm and the alcohol was intoxicating so no one really cared.

After the Roman Empire fell, the Christian Church took over and began to celebrate The Solemnity of All Saints on 1 November, and therefore named the night before All Hallows' Eve, the evening before All Saints' Day. So to Christians, Halloween is to All Saints' Day what Christmas Eve is to the birthday of Jesus, their Saviour.

Notice that we're still celebrating dead people here. Christianity just limited it to good dead people and said to hell with everyone else. Still, the actual celebrations of bonfires and drinking heavily and dressing up in silly costumes continued. Those who are really into partying never really care why they're partying unless it makes good small talk while you're mingling. The Christians were satisfied, and everyone else could get drunk and frighten each other with scary stories, and everyone lived happily ever after.

Source - BBC website