At Christmas Jesus will be born in our hearts, for at that time the anniversary of His birth will be celebrated. He refuses nothing, to the prayer of the Church, His spouse, and thus He will grant to our souls the same graces which He gave the shepherds and the wise Kings.
Christ will come again also, at the end of all time, to "condemn the guilty to the flames, and to call the just with a loving voice to heaven".
The whole of today's mass is a preparation for this double Advent of mercy and justice. Some parts of it can be applied equally to either (e.g., the Introit, Collect, Gradual, Alleluia), while others refer to our Divine Redeemer's lowly birth, and others again, (e.g. the Epistle and Gospel), to His coming in the splendor of His power and majesty. The same welcome will be given to us by our Lord when He comes to judge us, as we give to Him now when coming to redeem us. Let us prepare for the Christmas feast by holy prayers and aspirations and by reforming our lives, that we may be ready for that last great assize upon which depends the fate of our soul for all eternity. And all this with confidence, for those "who wait upon the Lord will never be confounded" (Introit, Gradual, Offertory).
In former times, on this First Sunday of Advent, all the people of Rome made the station at the Basilica of St. Mary Major, to assist at the solemn mass which the Pope celebrated, surrounded by his clergy. This particular Church was chosen because it is Mary who gave us Jesus and because relics of the crib in which the Blessed Mother placed her Divine Child are preserved in this Church.
Source: Dom Gaspar Lefebvre, OSB, 1945, adapted and abridged.