"It was on the stroke of midnight of Samhain's eve. The moment when the veil between the worlds are at its
Camelot's celebrations for Samhain.
thinest..." -Gaius (The Darkest Hour)
Samhain is a religious celebration and a day of great magical importance to the Old Religion. During Samhain people praise to their deads and everyone who departed. It's the End of Year Eve.
Celebrations
During Samhain the people of Camelot would prepare a feast to mourn their deceased and those who departed. The feast would be preceded by a speech from the current ruler before a toast (The Darkest Hour).
To the practitioners of the Old Religion, the High Priestesses would gather in the Isle of the Blessed where they would sacrifice a living human to tear 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 sorceresses can tap into for a limited time.
History
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 Christianised into the All Hollow's Eve, which was secularized into the Halloween.
Currently neopagan religions, specially those who claim Celtic and magic practise, such as Wicca, use it to symbolise to End of the Year, which was Samhian place in the religious calendar.