All Saints' Day, Nov. 1, feast of the Roman Catholic and Anglican churches, the day God is glorified for all his saints known and unknown. Roman Catholics are obliged to hear Mass on this day. In medieval England it was called All Hallows; hence the name Halloween (Hallows' eve) for the preceding day (Oct. 31).
From the Compton's Family Encyclopedia, 1991 Edition: Customs and superstitions gathered through the ages go into the celebration of Halloween, or All Hallows Eve, on October 31, the Christian festival of All Saints. It has its origins, however, in the autumn festivals of earlier times.
The ancient Druids had a three-day celebration at the beginning of November. They believed that on the last night of October spirits of the dead roamed abroad, and they lighted bonfires to drive them away. In ancient Rome the festival of Pomona, goddess of fruits and gardens, occurred at about this time of year. It was an occasion of rejoicing associated with the harvest; and nuts and apples, as symbols of the winter store of fruit, were roasted before huge bonfires. But these agricultural and pastoral celebrations also had a sinister aspect, with ghosts and witches thought to be on the prowl.
Even after November 1 became a Christian feast day honoring all saints, many people clung to the old pagan beliefs and customs that had grown up about Halloween.
Some tried to foretell the future on that night by performing such rites as jumping over lighted candles. In the British Isles great bonfires blazed for the Celtic festival of Sambain. Laughing bands of guisers, young people disguised in grotesque masks, carved lanterns from turnips, and carried them through the villages.
In the United States, children carved faces on hollowed-out pumpkins and put lighted candles inside to make jack-o'-lanterns. Halloween celebrations today reflect many of these early customs. Stores and homes display orange and black figures of witches, bats, black cats, and pumpkins. People dressed in fanciful outfits go to costume parties, where old-fashioned games like bobbing for apples in tubs of water may be a part of the festivities. Children put on costumes and masks and go from house to house demanding "trick or treat."