Read this passage from the Bible first:
At that time the Roman emperor, Augustus, decreed that a census should be taken throughout the Roman Empire. 2 (This was the first census taken when Quirinius was governor of Syria.) 3 All returned to their own ancestral towns to register for this census. 4 And because Joseph was a descendant of King David, he had to go to Bethlehem in Judea, David’s ancient home. He traveled there from the village of Nazareth in Galilee. 5 He took with him Mary, to whom he was engaged, who was now expecting a child.
Luke 2: 1-5
Now listen to the Christmas Carol - "Once in royal David's city":