“She will give birth to a son,” the angel tells Joseph, “and you are to give him the name Jesus, because he will save his people from their sins” (Matthew 1:21).
One thinks here of Abraham and Sarah longing for a son, of Hannah longing for a son, and of Mary longing for a son — Luke 1:46-55. Each of these longed for a son for different reasons, but Mary’s longing stands out as the longing for a son who would be Messiah. She longed, as did the rest of the Anawim (the pious poor) of her day, for the Messiah to come and to ransom captive Israel and to set the people free to worship in holiness and righteousness, to give the people peace and to establish justice.

So, the angel tells Joseph that Mary will give birth to a miracle-created baby, a son.
This son, Joseph learns, will be King Messiah, son of David and son of God.
Think of Mary being told this by the (same) angel:
?Do not be afraid, Mary, you have found favor with God. 31 You will be with child and give birth to a son, and you are to give him the name Jesus. 32 He will be great and will be called the Son of the Most High. The Lord God will give him the throne of his father David, 33 and he will reign over the house of Jacob forever; his kingdom will never end? (Luke 1:30-33).
Jesus is Son of…
Abraham
David
of God
Of the Most High
of Mary
of the Anawim
of the Priestly Leaders (through Zechariah, father of John the Baptist)
of Joseph
of the Righteous Ones
of the Exiled Generation
All these figures, promises, and experiences flow from Israel and God into Mary and Joseph to give birth to a Son who will embody it all. Here, in this Christmas word “son” we see all of history recapitulated and salvation embodied in a human body.