Here are the readings for the fourth Sunday in Advent:
http://www.usccb.org/nab/122108.shtml
This week we hear the story of the Visitation; the angel Gabriel appears to Mary and tells her that she is to be the mother of Jesus. Lots of attention is paid to Mary's obedience...but I think there's a lot more to it than simply doing as she was told.
Mary said YES. In spite of her fear and even though she didn't know HOW the things promised her could come to pass or how they would unfold over the months and years ahead.