It may seem strange that in a calendar with only one annual cycle of readings, two of the Sundays share virtually the same Gospel; and it may seem stranger still that these two Sundays occur consecutively. The Gospel for the Last Sunday of Pentecost, taken from St. Matthew, contains Christ's twofold description of the destruction of Jerusalem and of the world. That same speech reemerges the following week on the First Sunday of Advent.
Advent (from the Latin word for "coming") is generally considered to be the sober yet joyful time of preparation for the Lord's nativity, and rightfully so.
This is the beginning of the Church year that corresponds to the ages before Christ, when the world pined away in darkness, waiting for the Messiah. That is why the Masses of Advent allude to so many of Christ's predecessors: to Jacob, Juda, Moses, David, Michaes, Jeremiah, Ezechiel, Daniel, Joel, Zachariah, Habakuk, Hoseah, Haggeus, Malachiah, and especially to Isaiah, St. John the Baptist, St. Joseph, and the Blessed Virgin Mary. It is why the collects during Advent do not mention Jesus by name, a literary device that symbolizes His absence from this period of history. It is also why the closer we come to the Feast of the Nativity, the more we are called by the liturgy to reflect on the events that led up to it, e.g., the Annunciation, the Visitation, and so on. And it is why the season of Advent is marked by an ever greater urgency in its prayers, begging the Lord to come and tarry not.
Yet like the closing Sundays after Pentecost, which strike a predominantly apocalyptic note, the season of Advent also goads us to prepare for the glorious Second Coming of the Lord at the end of time. That is why the last and first Sundays of the liturgical year have the same divine admonition: one is picking up where the other left off. This eschatological focus remains throughout Advent, despite the season's increased focus on the Christ Child: in fact, during Advent the traditional Roman Rite frequently speaks of both in the same breath. This double commemoration of the first and second Comings makes sense, since the prophets themselves never distinguished between the two. Yet there is a more profound reason behind the conflation. The Church is teaching us that in order to be ready for the Lord's triumphant return as Judge of the living and the dead, we must prepare as our holy fathers once did for His nativity. The lessons we learn from the season of Advent are to be applied throughout our lives in preparation for our soul's Bridegroom. By liturgically preparing for the Nativity of our Lord, soberly and vigilantly, we prepare ourselves for the Final Judgment.