Samhain is the core of Halloween, and a special demon. So special that, he can only leave Hell when summoned by two powerful witches through three blood sacrifices over three days, the last on the final day of harvest, which is the 31st of October, Halloween. Once Samhain has risen, he is able to raise ghosts, zombies, and Ghouls and unleash them onto the world.
Background
When he reigned on Earth on Halloween night, people kept their children in that night, they wore masks to hide from him, carved pumpkins to worship him, and left sweets at their doors to appease him.
Samhain, once summoned by Tracy Davis and Don Harding, possesses the dead body of 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 has appeared on the show before. His unique eye color may mean that he is somewhere between the white eyed demons, such as Lilith, and the black eyed demons.
Powers
In the episode "It's the Great Pumpkin, Sam Winchester" Samhain shows multiple abilities. Sam tells Dean that Samhain, if risen, will 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, its stated the longer he is free, the more dangerous monsters he would summon. Also, he seems to share with the ability to have the fiery white light emanate from his palms.
Weaknessess
It is possible that he can be wounded by Ruby's knife, since he made sure to get rid of it. He also cannot see people if they are wearing masks and costumes. Since he is a demon, he can be excorised by Sam, although he was more resilent to sam's powers than a average demon.