Advent originated as an annual period of repentance focused on preparation for Judgment Day. It has even been suggested that the liturgical hymn Dies Irae, that magnificent masterpiece which was for so many centuries sung at funeral Masses, may perhaps have been originally composed for use in Advent - its somber sentiments intended to concentrate our attention of Christ's final coming at the end of time as judge of all the world.
We will do that waiting - in what we might call “liturgical time” - by looking back, to get to the future. I sometimes think of Advent as an ecclesiastical version of the Macy’s Thanksgiving Day Parade, which ends with Santa Claus, thus formally introducing us to Christmas. Thus, the 4th Sunday of Advent will recall Jesus’ conception in his Virgin Mother’s body. The 2nd and 3rd Sundays, however, will recall the adult Christ’s public appearance on the historical stage as announced by John the Baptist, challenging us to recognize Jesus, here and now, in the present time, between Christmas and the end. Finally (but at Advent's beginning), the 1st Sunday puts past and present in perspective, focusing on Christ’s final coming, when (as we say in the Creed) he will come to judge the living and the dead.