Halloween’s origins date back to the ancient Celtic festival of Samhain (pronounced sow-in). The Celts, who lived 2,000 years ago in the area that is now Ireland, the United Kingdom and northern France, celebrated their new year on November 1. This day marked the end of summer and the harvest and the beginning of the dark, cold winter, a time of year that was often associated with human death. Celts believed that on the night before the new year, the boundary between the worlds of the living and the dead became blurred. On the night of October 31 they celebrated Samhain, when it was believed that the ghosts of the dead returned to earth. In addition to causing trouble and damaging crops, Celts thought that the presence of the otherworldly spirits made it easier for the Druids, or Celtic priests, to make predictions about the future. For a people entirely dependent on the volatile natural world, these prophecies were an important source of comfort and direction during the long, dark winter.
Straddling the line between fall and winter, plenty and paucity, life and death, Halloween is a time of celebration and superstition. People would light bonfires and wear costumes to ward off roaming ghosts. In the eighth century, Pope Gregory III designated November 1 as a time to honor all saints and martyrs; the holiday, All Saints’ Day, incorporated some of the traditions of Samhain. The evening before was known as All Hallows’ Eve and later Halloween. Over time, Halloween evolved into a secular, community-based event characterized by child-friendly activities such as trick-or-treating. In a number of countries around the world, as the days grow shorter and the nights get colder, people continue to usher in the winter season with gatherings, costumes and sweet treats.
History of Halloween:http://www.history.com/topics/halloween/history-of-halloween
All Saints' Day (in the Roman Catholic Church officially the Solemnity of All Saints and also called All Hallows or Hallowmas), often shortened to All Saints, is a solemnity celebrated on 1 November by parts of Western Christianity, and on the first Sunday after Pentecost in Eastern Christianity, in honor of all the saints, known and unknown.
In Western Christian theology, the day commemorates all those who have attained the beatific vision in Heaven. It is a national holiday in many historically Catholic countries. In the Roman Catholic Church, the next day, All Souls' Day, specifically commemorates the departed faithful who have not yet been purified and reached heaven. Catholics celebrate All Saints' Day and All Souls' Day in the fundamental belief that there is a prayerful spiritual communion between those in the state of grace who have died and are either being purified in purgatory or are in heaven (the 'church penitent' and the 'church triumphant', respectively), and the 'church militant' who are the living. Other Christian traditions define, remember and respond to the saints in different ways.