Samhain (sa:win) is a Gaelic festival marking the end of harvest season. Such festivals were celebrated in many cultures globally. The boundary between this world and the otherworld could be easily crossed. #Halloween probably emerged from such festivals, especially Samhain. Food was left for visiting souls of the dead, & it was thought fairies visited this world more easily. People also went door to door in costume, reciting verses in exchange for food. The Church shifted All Saints Day to Nov. 1 & All Souls Day to Nov. 2. in the 9th century. All of these merged to become Halloween.
Happy Halloween!