Samhain is the name of a Celtic tradition honored as a festival to mark the end of the harvest season. Although pronounced phonetically as "sam:hane" in most Western cultures, its actual Gaelic pronunciation is"sɑːwɪn". It takes place between the autumn equinox and the winter solstice, and is celebrated from sunset on October 31st to sunset of November 1st. Modern traditions surrounding the holiday of Halloween originate from this festival.
Samhain has also been used to describe a demonic Celtic deity. The custom follows that by disguising oneself in a fearsome visage such as a mask or a costume, and leaving sweets on the doorstep to appease him, one might avoid being tormented by Samhain. This observance evolved into the tradition of "Trick or Treat", in which one gives out candy as a reward to children dressed in costume on October 31st.
On the CW Network television series Supernatural, the resurrection of Samhain was one of the sixty-six Seals of Lilith. The breaking of each seal progressively led to a time when Lucifer would walk free on Earth. The ritual and incantation required to summon Samhain involved powerful magic and could only be conduced once every six-hundred years. An ancient witch who had taken the name Tracy Davis offered up her own brother, a warlock named Don Harding, as part of a blood sacrifice. The soul of Samhain took possession of Harding's corpse and then killed Tracy Davis.
Samhain went to a nearby cemetery where he used his power to resurrect the dead, calling forth ghosts and ghouls. Sam Winchester fought against Samhain and was able to banish his spirit to Hell through his power of exorcism.