Yes. The Divine Mercy devotion is approved and practised throughout the Catholic Church. A 1959 notification of the Holy Office placed prohibitions on it, but on 15 April 1978 the Congregation for the Doctrine of the Faith declared that those prohibitions were no longer binding. St. Faustina Kowalska was beatified in 1993 and canonized on 30 April 2000, when St. John Paul II announced that the Second Sunday of Easter would be called Divine Mercy Sunday. In 2002 the Apostolic Penitentiary attached indulgences to the feast, and in 2020 her memorial was added to the General Roman Calendar.
St. Faustina's Diary is private revelation. Of private revelations, including those recognized by the authority of the Church, the Catechism (n. 67) says: "They do not belong, however, to the deposit of faith." Their role, it continues, is to help the faithful live more fully by Christ's definitive Revelation in a certain period of history. What the Church has done is permit and encourage the devotion, canonize the sister who recorded it, and name a Sunday of the liturgical year for the mercy of God.
The 1978 notification explains that the earlier prohibitions were lifted after original documents unknown in 1959 had been examined, circumstances had changed, and many Polish bishops had given their opinion. St. John Paul II himself said at the canonization that the message "is not a new message": it is the Gospel's own teaching on mercy.
Related questions, answered
Do Catholics have to believe St. Faustina's revelations?
They are private revelation. The Catechism (n. 67) says that private revelations "do not belong, however, to the deposit of faith"; they help the faithful live the Gospel more fully in a particular period. What the devotion expresses, that God is rich in mercy and that Christ forgives sins through His Church, is the common faith of Catholics.
Source: Catechism of the Catholic Church, 65–67 (private revelations, n. 67)
Is Divine Mercy Sunday an official feast of the Church?
Yes. At St. Faustina's canonization on 30 April 2000, St. John Paul II announced that the Second Sunday of Easter would from then on be called Divine Mercy Sunday throughout the Church. This did not add a new day to the calendar; it gave a name to the Sunday that closes the Easter octave.
Source: John Paul II: Homily for the canonization of Sr. Mary Faustina Kowalska (30 April 2000)
