Here is Stookey's explanation of the anomalies of Advent:
What may seem to be an anomaly is a very important theological point: the beginning of the liturgical year takes our thinking to the very end of things. For “end” means not only the “end of time,” but the central purpose or goal of creation. We are not aimlessly wandering in a wilderness, even though we may be tempted to think so. Rather history is headed somewhere by direction (though not dictation) from God. It is necessary that the liturgical year begin with this focus on a central, holy intention; for otherwise the story of Jesus which is about to be rehearsed from conception to birth to death and resurrection, may seem less than what it is: the deliberate fulfilling of divine purpose, worked out through historical process. Only this focus on the central purpose of God in history can keep the story of Jesus from falling into the superstitious or almost magical understandings that often afflict the Christian community, on the one hand, or into the trivializations and irrelevance that characterize secular interpretations, on the other hand.
The intertwining of an ultimate goal and of the historical process that leads to that end establishes the otherwise puzzling design of the Advent season. We start the Advent observances with the future: “The reign of God is coming. Prepare!” We end with the past: “Messiah will be born in Bethlehem. Rejoice!” As we proceed across the four Sundays of the season both motifs are sounded; but the volume of the future emphasis declines as the sound level of the emphasis on the past increases. Apart from coming to terms with this design, it is impossible to make sense of the fact that the Gospel readings of Advent begin with a mature Jesus teaching about the reign of God, and close with an unborn Jesus, still in Mary’s womb.
All of this is a reverberation of the sacred story, which to be understood aright, has to be read backwards. Just as the birth and ministry of Jesus as incomprehensible until we know of the Lord’s death and resurrection, so too the whole of the past is muddled unless we first have a grasp of the nature of the future.
Laurence Hull Stookey, Calendar: Christ’s Time for the Church, Abingdon Press, 1996.