The Catechism in Questions & Answers

124. What do we know about the birth of Jesus?

Caesar Augustus had ordered a census to be taken. For this, each person had to make his way back to “his own” city, in other words, the hometown of his family. For this reason Joseph, who was a descendant of David, went with Mary to the “city of David”, namely Bethlehem. There they were unable to find lodging. So it was that Mary must have given birth to her Son in a stable, because she placed Him in a manger: “And she brought forth her firstborn Son, and wrapped Him in swaddling cloths, and laid Him in a manger, because there was no room for them in the inn” (Luke 2: 7). From these events it is clear that God became human in very poor conditions.