The celebration of Halloween dates back to a mixture of Gaelic pagan traditions marking the end of the harvest known as Samain.
At this time of year they believed that the veil between the present and the afterlife was extremely thin and that the dead could come back to life inflicting diseases on both people and crops alike. To placate the spirits, people wore masks. As the Roman Empire made inroads into Gaelic territory, many of their traditions also mixed in with these pagan rites.
In the seventh century, the Church moved a holiday honoring God and all His saints to November 1, hoping to replace pagan festivals that took place on that date and involved the spirits of the dead. This celebration is known as the feast of All Saints or All Hallows. On All Hallows, the church remembers the "great cloud of witnesses" described in Hebrews 12 who have gone before us and are now with the Lord. In 1484, November 1 was declared a holy day of obligation: The faithful were required to attend Mass, in addition to fasting the night before. That is, they fasted on the Eve of All Hallows, from which we get the word Halloween.
But for most Americans and retailers, the celebration has become another feast at the altar of consumerism—Halloween is currently the sixth most profitable “holiday” (after Christmas, Mother's Day, Valentine's Day, Easter, and Father's Day) and our second most popular holiday (after Christmas) for decorating.
At best, it's an excuse to ask total strangers for candy. At worst, it's a celebration of the mindless paganism that our ancestors wisely turned their backs on. For most Christians though, it’s a missed opportunity for a much more meaningful celebration, remembering the saints that have gone before us.