December 21st is the first day of winter. It is the Winter Solstice. The time wherein the sun reaches its furthest decline at the horizon of the earth. Which is at the point of the Southern Cross in the night sky. It is symbolic of the decline and death of the sun or the death of Christ upon the cross.
We are currently in the period of time (December 23, 2017), wherein it appears, that the sun is stationary in this position. This stationary period lasts for three days. It's symbolic of the death of Christ and his body being in the tomb for three days.
December 25th is the first day that it is noticeable that the sun is beginning it's trek back north, bringing forth longer days and shorter nights. It is the sign of rebirth of the sun as it moves towards Spring and the generation of new life. This rebirth is symbolic of the birth of Christ who is the light of the world and the God of Creation. Jesus Christ being the God of Creation, designed the heavens and earth and he has declared, "all things bear record of me" (Moses 6:63). It is not surprising to see such symbolism of Christ within Creation itself. Merry Christmas!