Samhain

"It was on the stroke of midnight of Samhain's eve. The moment when the Veil between the worlds is at its thinnest..."

- Gaius Samhain is a religious celebration and a day of great magical importance to the  Old Religion. It marks the end of the harvest and of the lighter half of the year. Samhain also marks beginning of winter, or the darker half of the year. During Samhain it is traditional to celebrate those who have passed on. 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 had departed this life. The feast would be preceded by a speech from the current ruler before a toast.

As for the practitioners of the Old Religion, the High Priestesses would gather on 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 this universe. The sacrifice would unleash a vast deal of mystical power into the world which sorceresses could tap into for a limited time (The Darkest Hour).

Historicity
Samhain 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, Samhain was Christianised into the All Hollow's Eve, which was secularised into 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 Samhain place in the religious calendar.