"From that Heart, Sr. Faustina Kowalska, the blessed whom from now on we will call a saint, will see two rays of light shining from that Heart and illuminating the world: ‘The two rays,' Jesus Himself explained to her one day, ‘represent blood and water' (Diary, 299)."
Clearly, Divine Mercy Sunday is not a new feast established to celebrate St. Faustina's revelations. Indeed, it is not primarily about St. Faustina at all — nor is it altogether a new feast! As many commentators have pointed out, 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. In this way, it 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, point 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 Blessed 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."
Given the liturgical appropriateness of the title "Divine Mercy Sunday" for the Octave Day of Easter, therefore, the Holy See did not give this title to the Second Sunday of Easter merely as an "option," for those dioceses who happen to like that sort of thing! Rather, the decree issued on May 5, 2000, by the Sacred Congregation for Divine Worship and The Discipline of the Sacraments clearly states: "the Supreme Pontiff John Paul II has graciously determined that in the Roman Missal, after the title Second Sunday of Easter, there shall henceforth be added the appellation ‘or [that is] Divine Mercy Sunday'…".
Divine Mercy Sunday, therefore, is not an optional title for this solemnity; rather, Divine Mercy is the integral name for this Feast Day.