Samhain/Calan Gaeaf marked the end of the harvest season and beginning of winter or the 'darker half' of the year. Like Beltane/Calan Mai, it was seen as a liminal time, when the boundary between this world and the Otherworld thinned. This meant the Aos Sí (pronounced /iːˈʃiː/ ees-SHEE), the 'spirits' or 'fairies', could more easily come into our world and were particularly active. Most scholars see the Aos Sí as "degraded versions of ancient gods [. . . ] whose power remained active in the people's minds even after they had been officially replaced by later religious beliefs". The Aos Sí were both respected and feared, with individuals often invoking the protection of God when approaching their dwellings. At Samhain, it was believed that the Aos Sí needed to be propitiated to ensure that the people and their livestock survived the winter. Offerings of food and drink, or portions of the crops, were left outside for the Aos Sí. The souls of the dead were also said to revisit their homes seeking hospitality. Places were set at the dinner table and by the fire to welcome them. The belief that the souls of the dead return home on one night of the year and must be appeased seems to have ancient origins and is found in many cultures throughout the world. In 19th century Ireland, "candles would be lit and prayers formally offered for the souls of the dead. After this the eating, drinking, and games would begin".
Halloween or Hallowe'en (a contraction of All Hallows ' Evening ),  also known as Allhalloween ,  All Hallows' Eve ,  or All Saints' Eve ,  is a celebration observed in a number of countries on 31 October, the eve of the Western Christian feast of All Hallows' Day . It begins the three-day observance of Allhallowtide ,  the time in the liturgical year dedicated to remembering the dead, including saints (hallows), martyrs , and all the faithful departed.