Halloween has its origins in the ancient Celtic festival known as Samhain .The festival of Samhain is a celebration of the end of the harvest season in Gaelic culture, and is sometimes regarded as the "Celtic New Year". Traditionally, the festival was a time used by the ancient Celtic pagans to take stock of supplies and slaughter livestock for winter stores
L'Halloween est une fête traditionnelle à laquelle on attribue une lointaine origine celtique : il y a 2000 ans, les Celtes fêtaient, pendant sept jours, le changement d'année – appelé Samonios en Gaule, Samain en Irlande – aux environs du 1er novembre. Importée, pour finir, sur le continent nord-américain par les immigrants catholiques irlandais – l'Irlande ayant été évangélisée très tardivement au VIe siècle, c'est le pays où la fête de Samhain a subsisté le plus longtemps.
The term Halloween is shortened from All Hallows' Even (both "even" and "eve" are abbreviations of "evening", but "Halloween" gets its "n" from "even") as it is the eve of "All Hallows' Day",which is now also known as All Saints' Day. It was a day of religious festivities in various northern European Pagan traditions
(Source Wikipedia)