Samhain was considered to be a gateway not only from the land of the dead to the land of the living, but also between Summer and Fall/Winter. For the Druids, this was the last gasp (喘息, 氣喘)of summer (it was also the Celtic New Year), so therefore they made sure it went out with a bang before they had to button down (把...弄清楚)for the winter ahead.
They would dress up in bizarre costumes and parade through their villages causing destruction in order to scare off any recently departed souls who might be prowling (巡遊)for bodies to inhabit, in addition to burning animals and other offerings to the Druidic deities(神, 神性). It is also a popular belief that they would burn people who they believed to be possessed, but this has largely been debunked (揭穿, 拆穿假面具, 暴露)as myth.
This yearly festival was adopted by the Roman invaders, who helped to propagate (傳播, 宣傳)it throughout the rest of the world (and at that time, the Roman Empire was the world). The word "Halloween" itself actually comes from a contraction of All Hallows Eve, or All Saint's Day (November 1), which is a Catholic day of observance in honor of saints.