All Hallows Eve

When Christianity came to the Celtic areas it's believed the church wanted to replace this pagan ritual with their own. They had something around the same time of year called All Saints Day and All Souls Day which were days to honor the dead. Also with bonfires parties people dressing up as saints and angels and... Trick or treating. Originally poor citizens would beg for food and return for praying for the dead relatives of the person giving them the food. Eventually ordinary children would be going a soulling as they called it visiting houses in their neighborhood and being given food and money. So going a soulled became trick or treating. That's what people think.