The Oak King and the Holly King
"The God of the Waxing Year (who appears time and again in mythology as the Oak King) and the God of the Waning Year (the Holly King). They are the light and dark twins, each the others other self, eternal rivals eternally conquering and succeeding each other."
-- Janet and Stewart Farrar, Eight Sabbats for Witches (1981)
"The Year-DaemonVegetation Spirit or Corn God or whatever we call himwaxes proud and is slain by his enemy, who becomes thereby a murderer and must in turn perish at the hands of the expected avenger, who is at the same time the Wronged One re-risen."
-- Gilbert Murray, Euripides and His Age (1913)
"Representing the waxing year and the waning year, respectively, these Gods are actually one God who shows us different aspects of himself. He partners with the Goddess throughout the Year's Wheel. The Oak King rules from mid-WInter to Mind-Summer, while the Holly King rules from mid-Summer to mid-WInter. They ritually kill each other in order to claim their crowns, but always return at the appointed time. They echo earlier rituals when the Goddess ruled and the king may only rule beside her and at her biding and consent. Every few years or so the king must die so that a strong and fertile claimant may take his place in order to renew the land. The Oak King rules over growth and fecundity, while the Holy King rules over the time of withdrawal, contemplation, and rest from labors."
-- Galen Gillotte, Book of Hours: Prayers to the God
http://www.americanneopaganism.com/oakkingandhollyking.htm