I found this on a friend's facebook page. For all that went wrong with the Mists of Avalon miniseries, this part went just about right. Enjoy.
Update: Jenn pointed out that not everyone has read/watched Mists of Avalon, so here's a very condensed backstory: Arthur has to undergo a rite of kingship in order to claim his throne, the Priestess personifies the Goddess, the king personifies the Horned God, their mating symbolizes the creation of the world and the blessing of the land, and makes the king once with the land, thereby solidifying his kingship. Beltane is the Celtic festival of fertility.
There is also a subplot going on with Arthur, Guenivere and Lancelot, and a subplot with Morgainne and Acteon, both of which happen much later in the novel, but tie into the theme of the video.