Samhain

"When the veil between the worlds of the living and the dead are at it's thinest..."-Gaius, Darkest Hour Part 1

Samhain is a religious celebration and a day of great magical importance to the Old Religion. During the Samhain people praise to their deads and everyone who departed. It's the End of Year Eve

Celebrations
During the Samhain the people of Camelot would prepare a feast to mourne their deceased and those who departed. The feast would be preceded by a speach from the current ruler before a toast.

To the practioneers of the Old Religion, the High Priestesses would gather in the Isle of the Blessed where they would sacrifice a living human to thorn the veil between the worlds of the living and the dead, unleashing evil magical creatures into the world. The sacrifice would unleash a vast deal of mystical power into the world which sorcereresses can tap into for a limited time.

History
The Samhian is a pagan celebration where the spirits of the dead would came to the world to haunt the living. In some religions the religious caste would use the day to channel magical energy to cast spells. However as the religion of Christianity took over the pagan regions, the Samhian was christianizated into the All Hollow's Eve, which was secularized into the Halloween.

Currently neopagan religions, specialy those who claim celtic and magic practise, such as Wicca, use it to simoblize to End of the Year, which was Samhian place in religious calendar.