Jesus indicated several times to Sister Faustina that the Feast of Divine Mercy should be held on the second Sunday of Easter. Stated in her "Diary", "I desire that this image be displayed in public on the first Sunday after Easter. That Sunday is the Feast of Mercy. Through the Word Incarnate I make known the bottomless depth of My mercy" (No. 88). "I desire that the first Sunday after Easter be the Feast of Mercy" (No. 299). "...the first Sunday after Easter is to be the Feast of Mercy. On that day, priests are to tell everyone about My great and unfathomable mercy" (No. 570).
Before the Church confirmed the truth of Sister Faustina's revelation, the notes in her "Diary" had no binding power. It was widely debated whether the second Sunday of Easter could be named the Feast of Divine Mercy. Those who were accustomed to the traditional understanding of this Sunday also known as White Sunday as the ending of the paschal event cycle were opposed to renaming it the Feast of Divine Mercy. They thought that the new name would change the character of this Sunday. The most persistent proponent for introducing Jesus' recommendations was Father M. Sopocko, the confessor of Sister Faustina. He believed in the truth of these revelations and strived to support them theologically. He voiced his opinion many times on this topic before the Vatican II Council and after the introduction of the liturgical reform. He reasoned that it was not so much the private revelations as much as the liturgical spirit of the said Sunday which calls for instituting the Feast of Divine Mercy on the second Sunday of Easter (The spirit of liturgy of the second Easter Sunday, in the book, "Because His mercy is forever" (Poznan 1972, 377-392). John Paul II canonized Sister Faustina in 2000, instituted the Feast of Divine Mercy and spread it worldwide.
The decree, which instituted this holiday, explains that it does not change the character of the second Sunday of Easter but merely emphasizes what it has always celebrated. In other words, instituting the Feast of Divine Mercy on the second Sunday of Easter is a part of the journey to the origins of the paschal mystery of Christ. Only in this light do the miracles and mysteries appear in all of their beauty and abundance. The second Sunday of Easter culminates the events of the Last Supper, Golgotha and Easter morning. During this time, faithful believers set out to join in the Last Supper in the upper room, went with Jesus to Gethsemane, accompanied Him to start the Way of the Cross, suffered together with Him through the crucifixion, and finally rejoiced on the day of the Lord's resurrection. The paschal mystery of Jesus Christ shows how the salvation of man was accomplished, not by keeping records and settling debts, but it permeates throughout a person. God, by becoming a man, took upon Himself all of man's experiences including suffering and death. On this His love rests. His mercy is expressed also in this way, He did not destroy him who gave in to sin, but saved and elevated him to heavenly heights. The Resurrection of Jesus Christ demonstrates that the Love of God is more powerful than death, and God's mercy is greater than all human sins. The Magnificence of God is revealed in the fact that He allows man to participate in the acts of salvation of humanity.
The paschal "triduum" is the time when catechumens attain the grace of participating in godly life through baptism, confirmation and the Eucharist. These three sacraments give the paschal mystery of Christ presence in man's life. St. Paul writes, "Are you unaware that we who were baptized into Christ Jesus were baptized into His death? We were indeed buried with Him through baptism into death, so that, just as Christ was raised from the dead by the glory of the Father, we too might live in newness of life" (Rom 6:3-4). Through baptism, the Mercy of God enters a man, engulfs him and changes him. Each time, the Eucharist is an announcement of the death of the Lord until His return (1Cor 11:26). "Whoever eats My Flesh and drinks My Blood has eternal life ... remains in Me and I in him" (Jn 6:54-56).
In early centuries, after being baptized, catechumens put on white garments and participated in the liturgy of the Easter week until the next Sunday. This explains why this Sunday was called "white". The white color is a symbol of sainthood, cleanliness and innocence. This Sunday points the source and the results - Divine Mercy, which changes a sinner into a saint.
Bishop Edward Ozorowski - Dialogues on Divine Mercy