The early Christians incorporated the pagan celebration of winter solstice to the celebration of the birth of Christ. The solstice normally falls on the 21st of December and is followed by three days where the sun rises in the same place on the horizon. It is on December 25th that movement of the sun can be detected and so begins longer days of light. Thus, we celebrate the birth of Christ on December 25th.
Happy Christmas. May we all enjoy the light.