Halloween means Hallows' Evening. It is the evening before All Hallows' Day (now called All Saints'Day) , a Christian holiday, celebrated on the November 1st.
History traces Halloween back to the ancient religion of the Celtics. The Celts were the ancestors of the present-day Irish, Welsh and Scottish people. In the 5th century BC, in Celtic Ireland, summer officially ended on October 31st. On the November 1st, Celtic peoples celebrated the festival of Samhain,which marked the beginning of winter and the Celtic New Year. Celts thought the division between the natural world and the supernatural world became very thin and all time and space was abruptly suspended on October 31st, and then the spirits of the died would come back and move freely looking for living bodies to possess.