Halloween has origins in the ancient celtic festival known as Samhain (pronounced sow-in or sau-an), which 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 (pronounced kalan-geyf). The festival of Samhain celebrates the end of the "lighter half" of the year and beginning of the "darker half", and is sometimes(6] regarded as the "Celtic New Year". The name Halloween (or at least an Old English name which the modern term derives from), and many present-day traditions, derive from the Old English era. The celebration has some elements of a festival of the dead. 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 actual term 'Halloween' is said to have been originally originally been spelt Hallowe'en, and is shortened from All Hallows' Even – e'en is a shortening of even, which is a shortening of evening. This is ultimately derived from the Old English Eallra Halgena ?fen. It is now known as "Eve of" All Saints' Day, which is November 1.
In the 800s, the Church measured the day as starting at sunset, in accordance with the Florentine calendar. Although All Saints' Day is now considered to occur one day after Halloween, the two holidays were once celebrated on the same day.