Historians trace trick-or-treating to a few different ancestors.
One is the Celtic festival of Samhain, which marked the transition to the new year, and the end of the harvest and beginning of the winter.
The ancient Celts believed that during this night, the realms of the living and the dead overlapped and that spirits both good and bad could walk among the living.
To confuse and ward off the evil spirits, the Celts would sometimes impersonate them with costumes of white clothing and masks or blackface.
If they encountered a spirit during the feast, the costumed Celts would be mistaken for spirits and left alone.
As Christianity began to influence in the British, the old Pagan customs were Christianized and adapted to help sooth the Celts’ conversion.
Three Christian holidays—All Hallows’ Eve, All Saints’ Day and All Souls’ Day, together known as Hallowmas—were placed on the same days as Samhain.
All Hallow’s Eve eventually got shortened to Hallowe'en, and then Halloween, in conversation.
Going around the neighborhood for treats is most likely a form of souling, which started in the Middle Ages, also in the British Isles.
Soulers, mostly children and some poor adults, would go to door to door during Hallowmas and collect food or money in return for prayers said for the dead on All Souls’ Day.
A secular version of souling, called guising, eventually sprang up and is first recorded in Scotland in the 19th century.
Guisers went door to door and earned food treats or money by offering a small performance, like telling a joke or singing a song.
Some accounts of both these traditions make note of “fantastic costumes” that borrowed both from Samhain and British mummery.
(They also mention soulers and guisers carrying vegetable lanterns, precursors to the jack-o’-lantern.)
