The solemnity of the Sacred Heart sums up all the mysteries, joyful, sorrowful and glorious, of the Savior's life devoted to the love of God and men.
The Feast of Sacred Heart is celebrated on the Friday before this Sunday. But to give greater honor to the Most Sacred Heart and to allow the greatest number of the faithful to participate in the Mass and the Act of Reparation, the External Solemnity may be held on this Sunday, in accord with the rubrics of the Roman Missal (358a).
This feast is indeed placed on a height from which may be contemplated the redeeming labors of the Savior on earth and the glorious victories He will, by the working of the Holy Ghost, achieve in souls until the end of the world.
This solemnity does not relate to a single mystery of the Savior's life, but embraces them all; indeed the devotion to the Sacred Heart celebrates all the favors we have received from divine charity during the year (Collect), and all the marvelous things that Jesus has done for us (Introit, Tract, Alleluia). It is the feast of the love of God for men, a love which has made Jesus come down on earth by His Incarnation (Epistle), which has raised Him on the Cross for our Redemption and which brings Him down every day on our altars by transubstantiation, in order to make us benefit by the merits of His death on Calvary.
This solemnity has a character of reparation (Collect) demanded of us by the wounded Heart of Jesus and by His immolation in the Crib, on the Cross and on the Altar. Let us learn from the Heart of Jesus, in whose gentle and humble love we shall find rest for our souls (Alleluia).
Source: Dom Gaspar Lefebvre, OSB, 1945, adapted and abridged.