The Introit, Communion and Tract speak to us of Jerusalem compared to Mount Sinai by St. Paul in the Epistle for today. There, will the Christian people best raise their song of joy:
This song of joy is "Laetare" (Introit, Epistle) on account of the victory won by our Lord on the cross at Jerusalem, and there most easily, will be roused the memory of the heavenly Jerusalem, whose gates have been opened to us by the death of Christ.
It is for this reason, that formerly on this day, it was the custom […] solemnly to bless a rose, the queen of flowers. For, as we are reminded by the forms used for the blessing, in the traditional practice of Christian iconography, heaven is usually represented by a garden beautiful with flowers. For this blessing rose-colored vestments were used, and on this day a priest may celebrate Mass and Office in vestments of this color. Hence this custom was extended to the Third Sunday of Advent, Gaudete or "rejoicing" Sunday... And in its turn Laetare, also "rejoicing" Sunday, is a halting place in the midst of the Lenten observance.
"Rejoice, rejoice with joy," we are told in the Introit, for having died to sin with our Lord during Lent, we are shortly to rise with Him by the Paschal confession and communion. The Gospel speaks at one and the same time of the multiplication of the loaves and the fishes, symbols of the Eucharist and of Baptism, which were formerly received on the same occasion at Easter, and in the Epistle allusion is made to our deliverance by the sacrament of Baptism, which the catechumens formerly received at this season. And if we have had the misfortune to grievously offend almighty God, we shall recover our freedom by means of our Easter confession.
Source: Dom Gaspar Lefebvre, OSB, 1945, adapted and abridged.