The 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 the 1st November, in search of preys and victims. So, on the night of 31st October, they painted their faces and wore 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 was what gave rise to most of their fears.
Traditionally, during the time of the Samhain festival, the Celtic peasants used to take stock of their supplies and slaughter livestock for winters. The ancient Celts held the belief that on the said night, which is now observed as Halloween, the boundary between the living and the dead was dissolved and the dead souls became dangerous for the living, by causing them sickness and damaging their crops. Hence, the Celts arranged for bonfires and slaughtered livestock, throwing their bones into the raging inferno, to do away with the evil spirits. In Scotland, the young impersonated the dead with masks, veiled or blackened faces and dressed in white.
Initially, Celts used to place a skeleton on their windows on the night before Samhain, to represent the departed soul. They also used the head of a vegetable to frighten off evil, as they believed head to be the most powerful part of the entire body, possessing the spirit as well as knowledge. By 43 AD, Romans took hold of the Celtic territory. When brought together, the social interaction between the two societies and the cultural and traditional influence of the each society over the other resulted in the amalgamation of Samhain, the harvest festival mentioned above, and two Roman festivals - one to please Pomona, Goddess of fruits and trees and other Feralia, to commemorate the passing of the dead.
It is the festival associated with Pomona Goddess that gave rise to the custom of 'bobbing' apples on Halloween. 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, Jack-o'lanterns and carved pumpkin décor. The two main colors that have been traditionally associated with Halloween are orange and black, which continue to be used today as well. As for the Halloween costumes, they are usually those of monsters, ghosts, witches and devils, depicting traditional horror.