The 2000-year-old Celtic harvest and New Year festival, Samhain, was celebrated on 1st of November. Celts usually lived in Ireland, United Kingdom and northern areas of France. This festival marked the end of pleasant summers and the beginning of bitter cold. These people believed in evil spirits and witchcraft and thought that they get loose on Earth and roam about on the night before New Year in search of preys and victims. Thus, they painted their faces and wear terrible costumes to frighten away these ghosts and witches so that they would not harm them and their cattle and crops. The inexplicability of certain natural occurrences gave rise to most of their fears and Druids and the Celtic priests cashed in the opportunity by claiming that they could master these spirits and even make predictions and foretell the future with their help.
Though, Christians tried to celebrate 1st November as the peaceful and serene All Saints' Day but soon the festivities of Samhain caught favor among the Christian youths and Halloween became the day of big bonfires, grand parades, costumes, trick-or-treating and Jack-o'lanterns and carved pumpkin décor.
Halloween actually has dual origins. The first being a pre-Christian Celtic feast which is associated with the Celtic New Year. The Christian aspect of the holiday is the second. Pope Gregory IV, in 835, changed the celebration for martyrs (and later all saints) from May 13th to November 1st. All Saints Day is a holy day of obligation where we honor all saints, not just those canonized. Saints and holy people are called "hallowed" in England, thus All Saints Day is known as "All Hallow's Day." The evening before "All Hallow's Eve" or "holy evening." Later this was shortened to "Halloween." All Souls Day is celebrated on November 2nd. The church's reason for these feasts is to remember those who have died, whether canonized or not. We celebrate the "communion of saints," reminding us that the church is truly one and not limited to time or space. A Roman Catholic Pope decided on the day Halloween would fall. Halloween is the day before the Catholic religion celebrates All Souls'Day. Catholics can and do celebrate Halloween. Halloween is also Guy Fawkes Day.