Samhain was the Celtic celebration of the end of summer and the harvest that predated Halloween. It marked the transition to winter and darker days, as Samhain can be translated as “summer’s end.” The two day celebration occured from October 31st to November 1st. November 1st also marked the start of the New Year on the Celtic calendar. It was believed that this transition also blurred the lines between the living and dead. Large bonfires were built, and the Celtic people dressed up in costumes to honor the animals that they sacrificed to feed them through winter. It was a time of feast and festivities as well. The tradition of bobbing for apples would come out of the combination of Samhain and the Roman celebration of Pomona.
The Catholic Church sent missionaries to convert the Celts, eventually. In order to do this, they sought to replace or sanction some of the Celtic traditions by bringing them within the umbrella of Christianity. This is the best explanation for the reason that All Saint’s Day and All Soul’s Day would be set on November 1st and 2nd.