Hallowe’en is short for All Hallows Evening. Hallows is old English meaning "Holy" or "Saint". It is the evening of All Saints Day (with day counted from sun set to sun set making it is the evening before the day).

All Saints Day is a festival started about 1200 years ago by the Catholic Pope in Rome on November 1st as a time to celebrate all holy people who have died are are now, as Christians believe, with God. Later another day was added, November 2nd, called All Souls Day when people can remember and pray for everyone who has died.

This festival became very popular and spread all around Europe reaching The British Isles after about 50 years. The Irish, who had been Christians for 300 years, adopted the festival which replaced their existing one in the spring.

In the 16th century, some Christians, known as protestants, broke away from the Catholic church and formed various smaller churches some of which took a very anti-catholic position. One of these groups, the puritans, was very influential in the UK and USA. They were against Catholic festivals and tried to ban them. They attacked anyone who tried to continue the festivals calling them devil-worshippers, pagans, witches and the like. Just propaganda but it stuck.

This is why Americans and some British people celebrate Halloween as a time of witches, demons, ghosts and general nastiness – though they mostly don’t actually believe in any of it – except for a few extreme Christian sects.

The festival of All Saints & All Souls is till a very important feast in many other countries like Mexico, Latin America, Spain, Italy, Philippines, Hungary, Sweden etc. where is remains a day to celebrate and pray for the dead.
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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.

To commemorate the event, Druids built huge sacred bonfires, where the people gathered to burn crops and animals as sacrifices to the Celtic deities.

During the celebration, the Celts wore costumes, typically consisting of animal heads and skins, and attempted to tell each other’s fortunes. When the celebration was over, they re-lit their hearth fires, which they had extinguished earlier that evening, from the sacred bonfire to help protect them during the coming winter.

By A.D. 43, Romans had conquered the majority of Celtic territory. In the course of the four hundred years that they ruled the Celtic lands, two festivals of Roman origin were combined with the traditional Celtic celebration of Samhain.

there’s more on this site:http://www.history.com/content/halloween/real-story-of-halloween
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