“And when they were departed, behold, the angel of the Lord appeareth to Joseph in a dream, saying, Arise, and take the young child and his mother, and flee into Egypt, and be thou there until I bring thee word: for Herod will seek the young child to destroy him.”
Now when they had departed, behold, an angel of the Lord appeared to Joseph in a dream and said, Rise, take the child and his mother, and flee to Egypt, and remain there until I tell you, for Herod is about to search for the child, to destroy him. The second of four dream-revelations in this chapter (verses 12, 13, 19, 22). Joseph again receives the divine communication through dreams — the same vehicle as the patriarch Joseph in Genesis 37–41, whose dreams also led to Egypt and whose story of exile and return Matthew is consciously echoing. The command to flee to Egypt with the child and his mother is urgent: Herod is about to act. The one who will save his people from their sins must first be saved from the paranoia of a client king. Isaiah 49:24–25 asks who can rescue the prey from the powerful — the answer is the God who sends angels to warn in dreams.
Church Fathers on Matthew 2:13
Here Matthew omits the day of purification when the first-born must be presented in the Temple with the lamb, or a pair of turtle doves, or pigeons. Their fear of Herod did not make them bold to transgress the Law, that they should not present the Child in the temple. As soon then as the rumour concerning the Child begins to be spread abroad, the Angel is sent to bid Joseph carry Him into Egypt.
by this that the Angel appears always to Joseph in sleep, is mystically signified that they who rest from mundane cares and secular pursuits, deserve angelic visitations.
The first time when he would teach Joseph that she was lawfully espoused, the Angel called the Virgin his espoused 'wife;' but after the birth she is only spoken of as the Mother of Jesus. As wedlock was rightfully imputed to her in her virginity, so virginity is esteemed venerable in her as the mother of Jesus.