Several of the world's major winter holidays, including Christmas, were essentially set as a way of celebrating the return of the sun and the end of winter.
Today we know that this occurs on the Winter Solstice( about December 21-22 in the northern hemisphere), the time when the tilt of the earth's axis with respect to the plane of it's path around the sun causes the sun to start climbing higher into the sky at mid-day each day. Additionally, we have very precise methods of measuring when this occurs, including very precise clocks and various astronomical observation devices.
Did you ever wonder how early people determined the exact day for the Winter Solstice before they had modern day instruments? Those living several thousand years ago had very few methods available, and certainly did not have our modern day methods.
The simplest methods available involved observing the path of the sun through the sky, counting the number of days in a year (until the sun reached the same position), or observation of the stars.
Click on any menu link to see the theory, view articles about ancient solar observatories or to learn how to determine the date yourself in your own backyard.