Halloween is an annual holiday observed on October 31, which commonly includes activities such as trick-or-treating, attending costume parties, carving jack-o'-lanterns, bonfires, apple bobbing, visiting haunted attractions, playing pranks, telling scary stories, and watching horror films. The word Halloween is first attested in the 16th century and represents a Scottish variant of the fuller All-Hallows-Even ("evening"), that is, the night before All Hallows Day.
The day itself is typically associated with Samhain, a Celtic festival that marked the end of harvest. In Irish legends, the Cath Maige Tuired took place on Samhain. The deities Morrígan and Dagda met and had sex before the battle against the Fomorians. In this way, the Morrígan became a sovereignty figure and gave the victory to the Tuatha Dé Danann ("people of Danu"). After being defeated by the Milesians, these pagan deities became one with the sídhe, living on in popular imagination as fairies.
The Irish myths which mention Samhain were written in the 10th and 11th centuries by Christian monks. This is around 200 years after the Catholic church inaugurated All Saints Day and at least 400 years after Ireland became Christian.
Jack-o'-lanterns are typically carved pumpkins named after the phenomenon of strange light flickering over peat bogs, called jack-o'-lantern or will-o'-the-wisp (ignis fatuus, Latin "foolish fire"). These lights are also called ghost-lights or corpse candles. Sometimes the phenomenon is classified by the observer as a ghost, fairy, or elemental. Scientifically, the lights are explained as photon emissions produced by organic decay due to the oxidation of phosphine and methane.