In 2000, Pope John Paul II canonized Saint Faustina Kowalska and at the same time announced that Divine Mercy Sunday would be celebrated by the universal Church on the Sunday following Easter Sunday.
At the canonization John Paul II said: "Jesus told Sister Faustina: 'Humanity will not find peace until it turns trustfully to divine mercy.'"
How to Practice Divine Mercy
"I demand from you deeds of mercy, which are to arise out of love for Me. You are to show mercy to your neighbors always and everywhere. You must not shrink from this or try to excuse or absolve yourself from it.
I am giving you three ways of exercising mercy toward your neighbor: the first — by deed, the second — by word, the third — by prayer. In these three degrees is contained the fullness of mercy, and it is an unquestionable proof of love for Me. By this means a soul glorifies and pays reverence to My mercy.
Many souls ... are often worried because they do not have the material means with which to carry out an act of mercy. Yet spiritual mercy, which requires neither permissions nor storehouses, is much more meritorious and is within the grasp of every soul.
If a soul does not exercise mercy somehow or other, it will not obtain My mercy on the day of judgment. Oh, if only souls knew how to gather eternal treasure for themselves, they would not be judged, for they would forestall My judgment with their mercy" (1317).
What is Divine Mercy Sunday?
Among all of the elements of devotion to The Divine Mercy requested by our Lord through St. Maria Faustina Kowalska, the Feast of Mercy holds first place. The Lord's will with regard to its establishment was already made known in His first revelation to the saint, as recorded in her Diary. In all, there were 14 revelations concerning the desired feast.
Our Lord's explicit desire is that this feast be celebrated on the first Sunday after Easter. This Sunday is designated in "The Liturgy of the Hours and the Celebration of the Eucharist" as the "Octave Day of Easter." It was officially called the Second Sunday of Easter after the liturgical reform of Vatican II. Now, by the Decree of the Congregation for Divine Worship and the Discipline of the Sacraments, the name of this liturgical day has been changed to: "Second Sunday of Easter, or Divine Mercy Sunday."
Pope John Paul II made the surprise announcement of this change in his homily at the canonization of St. Faustina on April 30, 2000. There, he declared: "It is important then that we accept the whole message that comes to us from the word of God on this Second Sunday of Easter, which from now on throughout the Church, will be called ‘Divine Mercy Sunday.' "
By the words "the whole message," Pope John Paul II was referring to the connection between the "Easter Mystery of the Redemption" — in other words, the suffering, death, burial, resurrection, and ascension of Christ, followed by the sending of the Holy Spirit — and this Feast of Divine Mercy, the Octave Day of Easter, which fulfills the grace of atonement as lived through by Christ Jesus and offered to all who come to Him with trust.
Article from Aquinas and More