Origins of Halloween

Halloween stems from the medieval Celtic holiday Samhain, a Gaelic harvest festival held on October 31–November 1.  The festival marked the end of the harvest, the end of the "lighter half" of the year and beginning of the "darker half".

Samhain was the time to acknowledge that the livestock can no longer free forage on the vanishing summer grass, and was the time to take stock of the herds and grain supplies, and decide which animals would need to be slaughtered in order for the people and livestock to survive the winter.

Bonfires played a large part in the festivities. In Scotland, these bonfires were called samhnagan. With the bonfire ablaze, the villagers extinguished all other fires. Each family then solemnly lit its hearth from the common flame thus bonding the families of the village together. Often two bonfires would be built side by side, and the people would walk between the fires as a ritual of purification. Sometimes the cattle and other livestock would be driven between the fires, as well.

Autumn is a common season for observing the lights of the will-o'-the-wisp, a pale flame or phosphorescence sometimes seen over marshy ground at night. It is believed to be due to the spontaneous combustion of methane or other hydrocarbons originating from decomposing organic matter. These lights were often seen by travelers at night, especially over bogs, swamps or marshes, sometimes luring unweary travelers off the safe path, giving rise to folklores as to their actual origins being various prankster spirits. The phenomenon is known by a variety of names, including ignis fatuus (foolish fire), hinkypunk, hobby lantern, and jack-o'-lantern!

During this season, lanterns were carved from the plentiful turnips, and were used to recreate the mysterious jack-o-lantern light as an autumn decoration, and these lanterns were likewise sometimes called jack-o-lanterns. They were also presumably used to carry the flame from the samhnagan bonfire into the home. 

Association with All Saints
It was common for the Catholic church to schedule religious celebrations to coincide with the timing of festivals indigenous to cultures they were trying to conquer, as a means of allowing converts to continue to celebrate their favorite local customs but within the new context of Christianity. The fixing of the feast of All Saints, which originally was celebrated on May 13th or April 20th, to its current date of 1 November is first attributed to Pope Gregory III (731–741), and in AD 835 Pope Louis the Pious formally installed the festival on 1 November.

All Saints Day, also known as All Hallows Day, for early Christians was a time to pray for the souls of the dead trapped in purgatory. Since Samhain took place the day before All Hallows Day, Samhain came to be known as All Hallows Evening, or All Hallows-Eve, or Halloween. With the new association of praying for the dead, Samhain, or Halloween, adopted the new after-life associations; ghosts, ghouls, and then later other scary things of the night. The carved turnips used to carry the flames incorporated faces on them to be thought of as representing the souls of the people they prayed for. As the Catholic church continued to press followers to adopt the Christian traditions, they further began to associate the preceding ancient customs with evil and devilry, and encouraged the people to abandon the previous customs now classified as evil in favor of the customs the Christians deemed holy.

In the new Americas, pumpkins were not only more plentiful than turnips, they were softer, larger, and easier to carve, and so quickly replaced the turnip as the source of choice for jack-o-lanterns.

Trick-or-treating and guising

Trick-or-treating resembles the late medieval practice of souling, when poor folk would go door to door on Hallowmas (1 November), receiving food in return for prayers for the dead. Disgruntled beggars would often vandalize the homes of uncharitable homeowners who turned the beggars away empty handed, giving rise to the threat "trick or treat." Since such vandalism could bring upon the perpetrator the repercussions of the law, beggers often went in disguise (guising), giving rise to the more modern and friendly custom of children dressing up and trick or treating for candy.