Advent is not so much the preparation for Christmas as it is the preparation for His final coming, as you note. In my notes from a class I took from Dr. Pfatteicher a couple of summers ago I wrote the following: "First three Sundays: How do you know Jesus will return? The answer is given on the last Sunday: He came the first time. We should think of Bethlehem as a guarantee of the fulfillment of His coming." The best way to understand Advent is in terms of the fulfillment of prophesies. Thus, we know that at the end of time we can also be sure that all the prophesies of that time will also be fulfilled. It's a completion of the whole story. Past, present and future are collapsed and made a single whole. Advent reorients our sense of time.