Hallowe'en n : a shortened version of the name 'All Hallows Even'. It marks 'The Eve of All Saints' which occurs each year on the 31st of October. This is also the last night of the year in the early Celtic Calendar.
The modern holiday of Halloween has its origins in the ancient Celtic festival known as Samhain (Irish pronunciation: [ˈsˠaunʲ]; from the Old Irish samain). 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 pagans to take stock of supplies and slaughter livestock for winter stores. The ancient Gaels believed that on Octobe 31, the boundary between the alive and the deceased dissolved, and the dead become dangerous for the living by causing problems such as sickness or damaged crops. The festivals would frequently involve bonfires, where the bones of slaughtered livestock were thrown. Costumes and masks were also worn at the festivals in an attempt to mimic the evil spirits or placate them.