Samhain is observed from sunset on October 31st to sunset on November 1st. It is the celebration that is the origin of Halloween. Samhain was first observed by Celtic Pagans. Samhain marked the Celtic New Year, the end of summer, and the end of the harvest season.

Most American Halloween traditions were inherited from Irish and Scottish immigrants. Folklorists have used the name 'Samhain' to refer to Gaelic 'Halloween' customs until the 19th century. Since the later 20th century Celtic neopagans and Wiccans have observed Samhain, or something based on it, as a religious holiday.

Deities associated with Samhain, in Irish mythology, are the Morrigan and Crom-Cruach. While Morrigan is known to be the threefold goddess of the dead, theories abound about Crom-Cruach. In some legends he is said to be the personification of darkness; archeological evidence points to him being a fertility god.

At Samhain, held on November 1, the world of the gods was believed to be made visible to humankind, and the gods played many tricks on their mortal worshippers; it was a time fraught with danger, charged with fear, and full of supernatural episodes.