Historian Nicholas Rogers on the origin of All Hallows' Eve: while some folklorists have detected its origins in the Roman feast of Pomona, the goddess of fruits and seeds, or in the festival of the dead called Parentalia, it is more typically linked to the Celtic festival of Samhain, whose original spelling was Samuin. The name is derived from Old Irish and means roughly "summer's end".
A similar festival was held by the ancient Britons and is known as Calan Gaeaf.
The festival of Samhain celebrates the end of the "lighter half" of the year and beginning of the "darker half", and is sometimes regarded as the "Celtic New Year".
The ancient Celts believed that the border between this world and the Otherworld became thin on Samhain, allowing spirits (both harmless and harmful) to pass through. The family's ancestors were honored and invited home while harmful spirits were warded off. It is believed that the need to ward off harmful spirits led to the wearing of costumes and masks.