Winter Solstice is seen as the rebirth of light into the world throughout the Indo-European nations. Whether you are following a Celtic, Greek or any other Indo-European cultural practice modern Neo-Pagans follow a course of birth and rebirth each year that starts with the Winter Solstice.
Where Summer Solstice marks the longest day of the year and the decline of light in the world, Yule marks the longest night and the decline of darkness. Modern pagans celebrate Yule on December 21st or 22nd. As with the other solar high days the changing of light and darkness has an astrological basis, yet there is a spiritual side to this time of year as well that spans through nearly all indigenous European pagans.
During Winter Solstice the Earth is as its furthest point from the Sun in the orbital cycle. Also the Northern Hemisphere is as its maximum tilt from the Sun. During this time the as the Earth rotates the Northern Hemisphere, due to the tilt of the Earths axis, spends the longest amount of time in dark than any other time of year. Yet this time of year also marks the turning point of the orbiting cycle of the Earth and the changing of the tilt in the axis so that warmer weather and longer days begin to find their way back into the world.
The most popular of the spiritual or religious explanations for Yule is the struggle between the Holly and Oak Kings. In this myth the Holly King, the dying or shortened sun, battles with his son the Holly King, the newly born or growing sun. The battle ends with the dead of the Holly King, which in turn gives rule of the world over to the Oak King. This is a very popular belief among many Wiccans and Neo-Pagans.
Another way to look at Yule is a time to honor Apollo. Apollo was seen as a god of light and in many cases was associated with the sun in the Hellenic world. So it would be reasonable to use Yule as a time to honor Apollo. But there is another Greek deity that would be appropriate to honor during Yule or Winter Solstice, Helios. Helios was seen as a cycling deity that rose from the ocean in the morning and moved across the sky throughout the day in His chariot to finally sink again into the waters of the ocean at night. This, like many other myths, could very well be seen as the birth, life, and death of the god not only throughout the journey of a day but also throughout a year. Yule could be seen as the rebirth of Helios back into the world and Him starting His journey through the year in the great chariot riding across the sky.