It is Laetare Sunday. As Fr. Mark explains so well:Laetare Jerusalem: et conventum facite omnes qui diligitis eam: gaudete cum laetitia, qui in tristitia fuistis: ut exsultetis, et satiemini ab uberibus consolationis vestrae. (Psalm) Laetatus sum in his, quae dicta sunt mihi: in domum Domini ibimus. Gloria Patri.
Rejoice, O Jerusalem: and come together all you that love her: rejoice with joy, you that have been in sorrow: that you may exult, and be filled from the breasts of your consolation. (Psalm) I rejoiced at the things that were said to me: we shall go into the house of the Lord. Glory be to the Father.
Jerusalem is, according to the psalmist, “the dwelling of all joy” (cf. Ps 86:7). In Rome, where the Lenten liturgy is celebrated in ancient stational churches, the Mass of Laetare Sunday is set in the Basilica of the Holy Cross in Jerusalem. In that context, the great cry, “Jerusalem!” has a special resonance. You may ask why this basilica came to be called “in Jerusalem” when, in fact, it stands in Rome. In antiquity, it was called simply, “Jerusalem.” To go to the Basilica of the Holy Cross was to go “up to Jerusalem.” When, in the year 326, Saint Helena returned from the Holy Land, laden with relics, she had with her the most astonishing treasure: she had filled the entire hold of a ship with earth excavated from the holy places in Jerusalem. She had this sacred earth from Jerusalem deposited beneath the Sessorian palace that, enriched with relics of Our Lord’s blessed Cross and Passion, was to become her own church. Saint Helena’s church became “Jerusalem come to Rome.” (Read more.)Here is a meditation from The Liturgical Year by Abbot Gueranger, O.S.B.:
This Sunday, called, from the first word of the Introit, Lætare Sunday, is one of the most solemn of the year. The Church interrupts her lenten mournfulness; the chants of the Mass speak of nothing but joy and consolation; the organ, which has been silent during the preceding three Sundays, now gives forth its melodious voice; the deacon resumes his dalmatic, and the subdeacon his tunic; and instead of purple, rose-coloured vestments are allowed to be used. These same rites were practised in Advent, on the third Sunday, called Gaudete. The Church's motive for introducing this expression of joy into today's liturgy is to encourage her children to persevere fervently to the end of this holy season. The real mid-Lent was last Thursday, as we have already observed; but the Church, fearing lest the joy might lead to some infringement on the spirit of penance, has deferred her own notice of it to this Sunday, when she not only permits, but even bids, her children to rejoice!...
The blessing of the golden rose is one of the ceremonies peculiar to the fourth Sunday of Lent, which is called on this account Rose Sunday. The thoughts suggested by this flower harmonize with the sentiments wherewith the Church would now inspire her children. The joyous time of Easter is soon to give them a spiritual spring, of which that of nature is but a feeble image.
Hence, we cannot be surprised that the institution of this ceremony is of a very ancient date. We find it observed under the pontificate of St. Leo IX (eleventh century); and we have a sermon on the golden rose preached by the glorious Pope Innocent III, on this Sunday, and in the basilica of Holy Cross in Jerusalem. In the middle ages, when the Pope resided in the Lateran palace, having first blessed the rose, he went on horseback to the church of the Station. He wore the mitre, was accompanied by all the Cardinals, and held the blessed flower in his hand. Having reached the basilica, he made a discourse on the mysteries symbolized by the beauty, the colour, and the fragrance of the rose. Mass was then celebrated.
After the Mass, the Pope returned to the Lateran palace. Surrounded by the sacred college, he rode across the immense plain which separates the two basilicas, with the mystic flower still in his hand. We may imagine the joy of the people as they gazed upon the holy symbol. When the procession had reached the palace gates, if there were a prince present, it was his privilege to hold the stirrup, and assist the Pontiff to dismount; for which filial courtesy he received the rose, which had received so much honour and caused such joy. ~ Dom Gueranger's The Liturgical Year, Vol. V