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 3 1st. 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 ve ry thin and all time and space was abruptly suspended on October 3 1st, and then the spirits of the died would come back and move freely looking for living bodies to possess.
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Halloween (or Hallowe'en) is a holiday celebrated on October 3 1 . It has roots in the Celtic festival of Samhain and the Christian holy day of All Saints. It is largely a secular celebration, but some Christians and Pagans have expressed strong feelings about its religious overtones. Irish immigrants carried versions of the tradition to Nor th America during Ireland's potato famine of 1846. The day is often associated with the colors orange and black, and is strongly associate with symbols such as the jack-o'-lantern. Halloween activities include trick-or-treating, ghost tours, bonfires, costume parties, visiting haunted attractions, carving jack-o'-lanterns, reading scary stories, and watching horror movies.
There are more contents about Halloween on Wikipedia. You can directly enter the keyword Halloween in google and you will find the page of Wikipedia.
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This website has some information about Halloween in Chinese to help you understand the festival.
I wish you happiness! O(∩_∩)O~~