The angel came in the night because that is when lights are best seen and that is when they are most needed. God comes into the common for the same reason. His most powerful tools are the simplest.
Max Lucado, The Applause of Heaven
And there were shepherds living out in the fields nearby, keeping watch over their flocks at night. An angel of the Lord appeared to them, and the glory of the Lord shone around them, and they were terrified. But the angel said to them, “Do not be afraid. I bring you good news that will cause great joy for all the people. Today in the town of David a Savior has been born to you; he is the Messiah, the Lord. This will be a sign to you: You will find a baby wrapped in cloths and lying in a manger.
Luke 2: 8-12 NIV