Halloween, also known as All Hallows' Eve or All Saints' Day, originated as a celtic holiday that was celebrated by Druids, the priests of a religious order in ancient Gaul and Britain. The celebration marked the end of summer harvest season and pumpkins, cornstalks and other similar products of the earth were used in the feasting and merrymaking.
In the eighth century, the Catholic Church realized it was failing in efforts to keep Christians from celebrating the pagan holiday and decided to incorporate Halloween into the Christian calendar. The holiday would be celebrated on the first of November 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 the Christian celebration, including the tradition to dress up as ghosts, goblins, witches, fairies, elves and other mythical creatures.
Today, Halloween is celebrated in a number of countries around the world, including the United States, with customary practices that include trick-or-treating, costume parties, lighting bonfires and visiting haunted houses.
While many American non-Orthodox Jews do tend to celebrate the non-religious traditions of Halloween, halacha prohibits Jewish participation in the holiday.
The reasons for the Jewish prohibition vary. Mainly, Jews are forbidden by the Torah to partake in "gentile customs," a prohibition derived from Leviticus 18:3. This edict has been used by Jewish religious leaders as a source to determine the dress code and permissible behavior of the Jewish community throughout history.
Jews are also not allowed to partake in non-Jewish or idoltorous worship, per the Ten Commandments. Halloween, having both pagan and Catholic backgrounds, is deemed a gentile festival and is therefore forbidden to Jews.
That being said, despite Halloween's religious origins most Americans consider the holiday to be a national tradition - much like Thanksgiving - without the attachment of any religious significance. Many American Jews have adopted this interpretation of the tradition with the understanding that the holiday has only a secular meaning.
Rabbinically speaking, however, a holiday's origins do not simply disappear over time, so Halloween would still be considered a religious holiday- gentile in nature and ultimately against Jewish law.