The most ancient prayer to Mary we possess, from third century Egypt.
Four lines of shelter. Christians were praying these words while the Roman persecutions were still running.
When: In danger, in anxiety, over a sleeping child, at the end of Compline in many communities.
Why: It asks for one thing, protection, and grants Mary the title Theotokos, Mother of God, more than a century before the Council of Ephesus defended it. Praying it puts you inside the Church's longest memory.