Jews and Halloween
Halloween originated as a celtic holiday and was celebrated by Druids (priests of a religious order in ancient Gaul and Britain). The celebration marked the end of summer, and pumpkins, cornstalks, and similar products of the earth were used in the feasting and merrymaking.
In the eighth century, when the Church saw it would not succeed in weaning people away from celebrating the pagan holiday, it incorporated Halloween into the Christian calendar. The holiday would be celebrated on November the first as a day honoring all saints, hence the name All Saints' Day. The night before, October 31, was called "holy [hallowed] evening," and many of the old pagan Druid practices were retained in its celebration, including the dressing up as ghosts, goblins, witches, fairies, and elves.
While many American non-Orthodox Jews tend to celebrate Halloween on October thirty-first of each year, traditional halakhic ruling prohibits Jewish participation in this holiday.
The reasons for this prohibition vary. Mainly, as seen in Leviticus 18:3, Jews are forbidden to partake in "gentile customs". This statement has been used as a source to determine dress codes and permissible behavior for Jews throughout history. Jews are by law not allowed to partake in non-Jewish or idoltorous worship.
Halloween, having both pagan and, later, Catholic backgrounds is deemed a gentile festival and is therefore forbidden to Jews.
That being said, despite Halloween's religious origins, most Americans consider Halloween to be a national tradition without the attachment of any real religious meaning. As part of their assimilatory process, many American Jews have adopted this tradition as their own with the understanding that the holiday has taken on a secular meaning.
Rabbinically speaking, a holiday's origins cannot simply disappear over time and thus, Halloween would halakhically be considered a religious holiday- gentile in nature and ultimately against Jewish law.
Source: Kolatch, Alfred J. The Second Jewish Book of Why. Jonathan David Publishers, Inc.; Middle Village, New York, 1985; MyJewishLearning; BeingJewish.com