Why do we celebrate Christmas on December 25th?

There are two specific theories for why we use the date of December 25th for Christmas.

First, people and religions of the day celebrated some sort of holiday around that time. From Jewish Chanukah to Pagan Winter Solstice to Germanic Yule to Roman Dies Natalis Solis Invicti (Birth of the Unconquered Sun); the sheer number of celebration days with trees, decorations, yule logs, mistletoe and feasts seem to point to a season of celebration to which Christians added the birth of Jesus as a counter-cultural event and possibly even an escape from the pagan holidays for early believers.

December 25th was the Saturnalia Festival of emancipation, gift giving and the triumph of light after the longest night. The Christian sees the truth implicit in this pagan tradition that reflects: Christ the Light of the world, His triumph over the night of sin in
Luke 1:78-79 :

“…Because of the tender mercy of our God, by which the rising sun will come to us from heaven 79 to shine on those living in darkness and in the shadow of death, to guide our feet into the path of peace.”

The second theory centers around the date “accepted” by the Western Church of March 25 as the Annunciation or Immaculate Conception of Jesus in Mary’s womb. December 25 is 9 months later and thus celebrated as the birthday of Jesus.

Regardless of the possible reasons for the date, the church calendar was set in the West during Constantine’s reign while the Eastern Church held onto the date of January 6 for some time.