20 These things being so, Venerable Brethren, just as the rite of consecration, starting from humble beginnings, and afterwards more widely propagated, was at length crowned with success by Our confirmation; so in like manner, we earnestly desire that this custom of expiation or pious reparation, long since devoutly introduced and devoutly propagated, may also be more firmly sanctioned by Our Apostolic authority and more solemnly celebrated by the whole Catholic name. Wherefore, we decree and command that every year on the Feast of the Most Sacred Heart of Jesus, - which feast indeed on this occasion we have ordered to be raised to the degree of a double of the first class with an octave - in all churches throughout the whole world, the same expiatory prayer or protestation as it is called, to Our most loving Savior, set forth in the same words according to the copy subjoined to this letter shall be solemnly recited, so that all our faults may be washed away with tears, and reparation may be made for the violated rights of Christ the supreme King and Our most loving Lord.
Source: Miserentissimus Redemptor (Vatican.va)