I think we sometimes forget that Advent was the time Mary and Joseph traveled to Bethlehem.
From Nazareth to Bethlehem is about 90 miles. It would be like us traveling from Hanover to Philadelphia on foot, over mountains and hills.
Las Posadas commemorate Mary and Joseph finally reaching their destination and now must find a place to stay over.
These celebrations happen in peoples' homes and is supposed to be like Mary and Joseph going from door to door, knocking and hoping to be welcomed into a home.
They begin nine days before Christmas, making it a Novena as well.youtube.com/user/splagatzo/videos