The wheel of the year turns

 

Samhain (pronounced /ˈsɑːwɪn/ SAH-win or /ˈsaʊ.ɪn/ SOW-in) is a Gaelic festival marking the end of the harvest season and the beginning of winter or the "darker half" of the year. It is celebrated from sunset on 31 October to sunset on 1 November, which is nearly halfway between the autumn equinox and the winter solstice. Along with Imbolc, Beltane, and Lughnasadh it makes up the four Gaelic seasonal festivals. It was observed in Ireland, Scotland, and the Isle of Man. Kindred festivals were held at the same time of year in other Celtic lands.

Samhain is mentioned in some of the earliest Irish literature and is known to have pre-Christian roots. Many important events in Irish mythology happen or begin on Samhain. It was the time when cattle were brought back down from the summer pastures and when livestock were slaughtered for the winter. As at Beltane [think May Day], special bonfires were lit, deemed to have protective and cleansing power. Samhain (like Beltane) was seen as a liminal time, when the spirits or fairies  could more easily come into our world. The souls of the dead were also thought to revisit their homes. Feasts were had, at which the souls of dead kin were beckoned to attend and a place set at the table for them. Mumming and guising were part of the festival, and involved people going door-to-door in costume (or in disguise), often reciting verses in exchange for food. The costumes may have been a way of imitating, or disguising oneself from, any dangerous spirits.
In the 9th century, the Roman Catholic Church shifted the date of All Saints' Day to November1 , while November 2 later became All Souls' Day. Over time, traditions associated with Samhain and All Saints'/All Souls' merged and helped to create the modern Halloween customs most of us know in the United States. Needless to say, in places where the Gaelic influence was less important, other traditions prevail.
 
Since the latter 20th century, Celtic neopagans and Wiccans have observed Samhain, or something based on it, as a religious holiday. Neopagans in the Southern Hemisphere often celebrate Samhain at the other end of the year -- April 30 to May 1, when those in the north are celebrating Beltane.
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Damien's note:  Daniel's niece has recently pledged (?) a Wiccan sorority on campus, and we are learning more about Neopaganism as a result. I already teach a course on unusual belief systems, but it is fascinating to be in direct contact with actual believers.