How did the practice of trick-or-treating develop on Halloween?

By the ninth century, Christianity had spread into Celtic lands, where it gradually blended with and supplanted older pagan rites. In 1000 A.D. the church designated November 2 as All Souls’ Day, a time for honoring the dead. Celebrations in England resembled Celtic commemorations of Samhain, complete with bonfires and masquerades.  

Poor people would visit the houses of wealthier families and receive pastries called soul cakes in exchange for a promise to pray for the souls of the homeowners’ dead relatives. Known as "souling," the practice was later taken up by children, who would go from door to door asking for gifts such as food, money and ale

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there are isolated pockets of farming, generally around rivers and oases, which are used for irrigation.