The Ceremony of the Sun is a daily ritual of the Skeksis. Lead by skekZok the Ritual Master, the Skeksis gather around the Dark Crystal as the first sun climbs to its peak, sending light streaming down the Great Shaft in the center of the Castle of the Crystal.
The Skeksis learned how to draw power from the sun, and each day they replenish themselves - cheating death - through the power of the Dark Crystal.
For a time, the Ceremony of the Sun was enough to provide nourishment to the Skeksis, but then it started to fail. This lead directly to skekSo ordering skekTek to find a way to replenish the dying bodies of the Skeksis. This would lead to the draining of essence of Gelfings and eventually the Age of Resistance.
Later, the Ceremony of the Sun would work again, helping the Skeksis cheat death again until the healing of the Crystal.