To commemorate the birth of Jesus Christ
It is said that Jesus was conceived by the Holy Spirit and was born of the Virgin Mary.
Then Gabriel, an angel of God, told Joseph in a dream not to take Mary away because she was pregnant but to marry her and name the child Jesus. This meant that he should save the people from their SINS.
When Mary was about to give birth, the Roman government issued an order that all the people must declare their household registration in Bethlehem.
Joseph and Mary had to obey.
When they arrived in Bethlehem, the sky has been faint, but two people did not find the hotel to spend the night, only a stable can temporarily live.
Just then Jesus was about to be born.
So Mary alone gave birth to Jesus in a manger.
Posterity to commemorate the birth of Jesus, set December 25 for Christmas, every year at mass, to commemorate the birth of Jesus.