From the Celtic tradition, comes a pair of ancient pagan characters who fight twice a year, once on Midsummer (about June 21) and again on the Winter Solstice (about Dec 21). The spirits of the Holly King and the Oak King come from two of the Druids' most sacred trees: Holly and Oak. The Oak King is the lord of the waxing half of the year. He governs the Celtic earth from December to June. The Holly King is the lord of the waning half of the year. He governs the Celtic earth from June to December.
The battle between the two figures occurs on the winter solstice when the Oak King kills the Holly King at Yule and takes his place. The Oak King then reigns until Litha (midsummer) when the two battle again. The Holly King wins in June then reigns until Yule (winter solstice) in December.
The Holly King we all know as the present day Santa Claus. He wears red and bears holly leaves and berries in his hat. He drives a team of eight deer (or reindeer) because deer were highly sacred to the Celtic Gods. The number eight represents the eight sabbats of the solar calendar.
Many of us would recognize the Oak King as the modern-day "Baby New Year". He is the new, fresh and young child-god that beckons mother nature to re-new herself as he brings the warm rays of the sun back.
Mistletoe is sacred to the Oak King (and a symbol of love and procreation--hence the kissing under the mistletoe tradition). Mistletoe grows high in the branches of oak trees. The two plants, holly and mistletoe, are traditional to the Yule season and represent the battle that takes place between the two forces of nature. That mistletoe is common at this time of year also illustrates that although the Oak and Holly Kings are mortal enemies twice a year they are also two sides of a whole. One can't exist without the other and each respects the other despite their confrontational relationships.