Within the Sylvan Tradition, the God is considered to be a duality, separated into darkness and light. Part of the reason for this is his association with the sun. Where the Goddess is associated with the lunar cycle, the God is associated with the solar cycle. If we use the Summer and Winter Solstices as the dividing lines of the year, we find that the year is equally separated into light and dark portions. From Winter Solstice to Summer Solstice, the hours of daylight are slowly growing; each day the light grows a little stronger. At Summer Solstice (Midsummer), the hours of daylight begin to slowly fade, the darkness growing until it reaches the pinnacle of its strength at Winter Solstice (Yule).
Similar to the symbolism of mainstream Wicca, the Sylvan Tradition associates one aspect of the God (the Oak King) with the light portion of the year and the other aspect (the Holly King) with the dark half. This duality can successfully be used as a tool for personal and spiritual growth.
The Oak King begins his reign at Yule. The symbolism of the sun (or a sun god) being reborn at Summer Solstice is such powerful imagery that a number of cultures around the world have adopted it into their early beliefs and it is the most common reason given for the association of Jesus's birth being on December 25th (which on early calendars fell as close to Winter Solstice as a fixed date can). The Oak King is associated with the things of light, not only noble emotions and love, but also vegetation (as plants grow within his reign).
The Holly King takes over the year at Midsummer, slaying his twin. The second half of the year is a time of magick and death as the sun slowly fades. Mirroring the Oak King's association with vegetation, the Holly King is connected to animal life. He is also representative of shadows, both within and without us, and with death, reminding us that death is not something to be feared, but a natural part of life's cycles.
Within magick, the God is not only associated with the sun, but also with the right side of the altar. Tools that have phallic symbolism (wands, athames, etc) or have sharp angles are usually associated with the God.