Offerings of food and drink, or portions of the crops, were left outside for the Aos Sí.
It is widely believed that many Halloween traditions originated from Celtic harvest festivals which may have pagan roots, particularly the Gaelic festival Samhain, and that this festival was Christianized as Halloween.
Halloween activities include trick-or-treating (or the related guising), attending Halloween costume parties, carving pumpkins into jack-o'-lanterns, lighting bonfires, apple bobbing, divination games, playing pranks, visiting haunted attractions, telling scary stories and watching horror films.
In many parts of the world, the Christian religious observances of All Hallows' Eve, including attending church services and lighting candles on the graves of the dead, remain popular, In Scots, the word "eve" is even, and this is contracted to e'en or een.
Over time, (All) Hallow(s) E(v)en evolved into Hallowe'en.
Although the phrase "All Hallows'" is found in Old English "All Hallows' Eve" is itself not seen until 1556.
Jack Santino, a folklorist, writes that "there was throughout Ireland an uneasy truce existing between customs and beliefs associated with Christianity and those associated with religions that were Irish before Christianity arrived".
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.
They included apple bobbing, nut roasting, scrying or mirror-gazing, pouring molten lead or egg whites into water, dream interpretation, and others.
Historian Nicholas Rogers, exploring the origins of Halloween, notes that 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, which comes from the Old Irish for "summer's end".
A kindred festival was held at the same time of year by the Brittonic Celts, called Calan Gaeaf in Wales, Kalan Gwav in Cornwall and Kalan Goañv in Brittany; a name meaning "first day of winter".