It’s really very simple: Christmas is the feast of the incarnation and the season following that event. Advent is the recognition that we need a savior and the longing for that savior to come, according to God’s promises.
Christmas is joyous, but the joy comes after weeks of waiting, watching, lamenting and calling upon God. Advent is that season of waiting; of looking for the signs and promises of the savior in the scriptures and in the world.
That distinction should save us. We think we can manufacture our own salvation by going shopping. Advent says we cannot save ourselves, that only God can save us and that in his own time and in his own way.