I desire that the Feast of Mercy be a refuge and shelter for all souls, and especially for poor sinners. On that day the very depths of My tender mercy are open. I pour out a whole ocean of graces upon those souls who approach the fount of My mercy. The soul that will go to Confession and receive Holy Communion shall obtain complete forgiveness of sins and punishment. On that day all the divine floodgates through which graces flow are opened. Let no soul fear to draw near to Me, even though its sins be as scarlet. My mercy is so great that no mind, be it of man or of angel, will be able to fathom it throughout all eternity. Everything that exists has come forth from the very depths of My most tender mercy. Every soul in its relation to Me will contemplate My love and mercy throughout eternity. The Feast of Mercy emerged from My very depths of tenderness. It is My desire that it be solemnly celebrated on the first Sunday after Easter. Mankind will not have peace until it turns to the Fount of My Mercy.-- Diary of St. Faustina, 699
In his homily at the canonization of St. Faustina on April 30, 2000, Bl. Pope John Paul II 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.'”
In all, there were 14 revelations concerning the feast. However, Divine Mercy Sunday is not a new feast established to celebrate Faustina's revela¬tions. Indeed, it is not primarily about Faustina at all — nor is it altogether a new feast! The Second Sunday of Easter was already a solemnity as the Octave Day of Easter; nevertheless, the title "Divine Mercy Sunday" does highlight and amplify the meaning of the day. It also recovers an ancient liturgical tradition, reflected in a teaching attributed to St. Augustine about the Easter Octave, which he called "the days of mercy and pardon," and the Octave Day itself "the compendium of the days of mercy."
Liturgically the Easter Octave has always been centered on the theme of Divine Mercy and forgiveness. Divine Mercy Sunday, therefore, points us to the merciful love of God that lies behind the whole Paschal Mystery – the whole mystery of the death, burial and resurrection of Christ – made present for us in the Eucharist. In this way, it also sums up the whole Easter Octave. As Pope John Paul II pointed out in his Regina Caeli address on Divine Mercy Sunday, 1995: "the whole octave of Easter is like a single day," and the Octave Sunday is meant to be the day of "thanksgiving for the goodness God has shown to man in the whole Easter mystery."
source: The Marians of the Immaculate Conception, http://thedivinemercy.org
see also: Apostles of Divine Mercy