Today the O antiphon recalls the image of a key. The key is a key opens a jail cell. The jail cell it opens is the jail in which humanity was imprisoned, not sin, but he punishment for sin. This key is said to open the prison of "darkness and the shadow of death."
Once more today's antiphon links Christmas and Easter. It reminds us that, for the Christian, the arc of human existence is not from birth to death, not womb to tomb, but from conception to eternal life. God does not create human life to see it die, but wills that each of us should live forever.