The Demon Samhain in Supernatural
The demon Samhain can be summoned by a powerful witch by "3 blood sacrifices over 3 days, the last on the final day of harvest", which is the 31st of October, Halloween's Eve. Once Samhain has risen, he is able to raise ghosts, zombies and ghouls and unleash them onto the world.
Samhain, once summoned by Tracy Davis, possesses the dead body of school teacher Don Harding. When he enters the body, the corpse's irises turn milky white, while his pupils stay black, marking him as a different kind of Demon than the once we have seen before.
Powers and Abilities
In the episode "The Great Pumpkin, Sam Winchester", Samhain has shown multiple abilities. Sam tells Dean that if risen, that he would have the ability to raise monsters, such as zombies and demons, to do his bidding. This is proven when Samhain locks a group of students in a mausoleum and zombies attack them. Also, he seems to share the same ability as Lilith as far as being able to have the firey white light come from his palm. Other than these abilities, not much more is known about his powers.
Background - The Myth of Samhain
The myth of Samhain, the "Celtic God of the Dead" was born in the 19th century by Christians, meant to "demonisize" the Pagan celebration of Samhain. The creation of Samhain as a Celtic God can be traced back to a book by Godfrey Higgins in 1827 in which he tried to prove that the druids originated in India (Higgins referenced Samana, a Hindu/Vedic deity, God of the Dead.) The first reference though came from a man called Vallency, who mentioned the name in the late 18th century.
The word Samhain is derived from Old Irish and refers to the first of November. Samhain is the Festival of the Dead and symbolizes the end of the summer and the Celtic New Year. Traditionally, it was the day of the last harvest and the time to take inventory of live stock to be slaughtered.
According to Celtic lore Samhain is the time of year when the boundaries between the worlds of the living and the dead become transparent, allowing supernatural beings to pass between the worlds. Usually, it's the time of year where ancestors were especially honored (see "All Saints' Day). The Samhain Festival involved bonfires, as well as costumes and masks to disguise the living and make them mimic the evil spirits that walked the earth on Samhain.
Many Halloween traditions come from Irish and Scottish Lore - the carving of the pumpkins and lighting candles inside of them, the disguising as supernatural or evil creatures, the fireworks and bonfires. Trick-or-Treating derives from the tradition of Mischief Night. At Hollywood parties, typical games are dunking for apples or eating strung up scones without the use of hands.