The Bible records three different occasions that an angel of the Lord appeared to Joseph. The first was to let him know Mary was still a virgin and she had not cheated on him. The second to warn him to take the family and flee to Egypt because Herod wanted to kill Jesus.

After the wise men were gone, an angel of the Lord appeared to Joseph in a dream. “Get up! Flee to Egypt with the child and his mother,” the angel said. “Stay there until I tell you to return, because Herod is going to search for the child to kill him.” English Standard Version

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. Matthew 1:20

But in a dream, an angel appeared to Joseph and told him to trust Mary. The angel also told Joseph that the child should be called Jesus. Having a vision in a dream from God was a sign of God's approval, so this would have made Joseph pay attention and do what the angel had said!

An angel of the Lord appeared to Joseph in a dream and he said; “Take Mary home as your wife.” When Joseph woke up, he did as the angel commanded him to do. Mary and Joseph ended up in Bethlehem where the birth took place.