Samhain (pronounced "sow-in") is a Gaelic festival that marks the end of the harvest season and the beginning of winter or the "darker half" of the year. It is believed to have Celtic pagan origins and is one of the four main festivals of the Gaelic calendar. Samhain was a liminal or threshold festival, when the boundary between this world and the Otherworld thinned, making contact with the aos sí (the spirits or fairies) more likely. The festival was not recorded in detail until the early modern era, but it is mentioned in the earliest Irish literature from the 9th century and is associated with many important events in Irish mythology.

Samhain was a crucial time of year, loaded with symbolic significance for the pre-Christian Irish. It was viewed as a borderline or liminal festival as the separation between summer and winter, lightness and darkness was blurred. The Celts believed that the veil between the living and the dead was especially thin on this day, allowing spirits of the dead to visit the living. Samhain was a mandatory celebration lasting three days and three nights where the community was required to show themselves to local kings or chieftains.

Today, Samhain is observed by some Wiccans and neopagans as a religious holiday. Celebrants believe that the barriers between the physical world and the spirit world break down during Samhain, allowing more interaction between humans and denizens of the Otherworld. Many rituals are associated with Samhain today, including dancing, feasting, taking nature walks, and building altars to honor ancestors.