Wikipedia: Halloween

Halloween is a holiday celebrated in much of the Western world on the night of October 31. It is associated with trick-or-treating, [ghost stories]?, pumpkins, jack o'lanterns, witches, [black cats]?, costumes?, and parties. Children often dress up in costumes and knock on neighborhood doors saying, "Trick or Treat" and receiving candy, originally in return for a joke, a song, or some other trick.

There are several traditional games associated with Hallowe'en without which no Hallowe'en party is truely complete. The most common is Ducking for Apples in which a tub or a large basin is filled with water in which apples float. The participants must remove an apple from the basin using only their mouths. Naturally everyone gets wet. Another common game involves hanging up treacle or syrup coated scones by strings. These must be eaten without using hands while they remain attached to the string, an activity which inevitably leads to a very sticky face.

History

Although modern Halloween is a secular holiday, it evolved from several earlier religious holidays, first Pagan and later Christian.

Its earliest roots are found in the Druid? harvest holiday which took place each year on October 31. After the crops were harvested, Druids in Britain would light fires and offer sacrifices of crops and animals. As they danced around the fires, the season of the sun passed and the season of darkness would begin. When the morning of November 1 arrived, the Druids would give an ember from their fires to each family who would then take them home to start new cooking fires. These fires were believed to keep the homes warm and free from evil spirits. A 3 day festival called Samhain (pronounced "sow-en") followed.

After the Romans colonialized much of Britain, elements of the Roman festival known as [Pomona Day]? were also introduced. Pomona Day was held on November 1, and is named for Pomona?, a Roman Goddess of fruits and gardens.

Need more details about Pomona Day and which elements of it became part of modern Halloween.

When Christianity eventually reached Britain (year?), conversion began among the local people, including Christianization of the old traditions. In the 7th century, Pope Boniface IV introduced All Saints' Day to replace the pagan festival of the dead. It was observed on May 13. Later, Pope Gregory III changed the date to November 1. October 31st became known as All Hallow Even, eventually All Hallow's Eve, Hallowe'en (still used), and then Halloween in the US.

Need more details about the specific practices which were performed.

The custom of trick-or-treating is thought to have evolved from the 9th-century European custom called souling. On November 2, All Souls Day, Christians would walk from village to village begging for "soul cakes" - square pieces of bread with currants. Beggars would promise to say prayers on behalf of dead relatives helping the soul's passage to heaven.

Irish emigrants from the Irish Potato Famine of 1845-46 brought with them the holiday of Halloween to the United States.

Religious Viewpoints

Some fundamentalist Christian groups consider Halloween a Pagan holiday because of these early Pagan origins, and refer to it as "The most evil day of the year", refusing to allow their children to participate. Among these groups it is believed to still have Satanic influences, as are many other Pagan practices.

Neopagans also do not practice Halloween, but for different reasons. Instead of rejecting it because of its Pagan origins, they rather embrace the earlier Pagan practice and celebrate a version of the older Celtic festival of Samhain.