Samhain

Samhain, (Celtic: “End of Summer”) also spelled Samain, in ancient Celtic religion, [is] one of the most important and sinister calendar festivals of the year. 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.

Encyclopaedia Britannica