Of course Sundays, especially Sundays of Easter, trump everything! This is our first clue that Divine Mercy is not just another devotion and a Polish one foisted on the whole Church by a Polish pope. Contrary to what many think, or at least act as if this is what they think, these liturgical intricacies were certainly not lost on John Paul II when he made his declaration.
In seeking to cultivate a simpler, less inward-looking Church, Pope Francis is not trying to bring about a flatter, more didactic, academic Church, like the one envisioned by many who complain about the dissonance Divine Mercy Sunday causes them to experience. One of the worst liturgical tendencies these days is to render all things in a flat, linear way. This is very unappealing to most people and smacks more than a bit of a Protestant ethos and the aesthetic to which such an ethos gives birth. Divine Mercy is at the heart of the Paschal Mystery, it is the reason for it! Besides, the Sacrament of Penance, being an extension of Baptism, is most assuredly an Easter sacrament. We celebrate this sacrament during the Easter Octave, not just during Holy Week.
With reference to St. Faustina, I turn to something important written by Hans Urs Von Balthasar in his book Two Sisters in the Spirit: Thérèse of Lisieux and Elizabeth of the Trinity:
In modern times, theology and sanctity have become divorced, to the great harm of both. Except in a few cases, the saints have not been theologians, and theologians have tended to treat their opinions as a sort of by-product, classifying them as spiritualité or, at best, as théologie spirituelle. Modern hagiographers have contributed to this split by describing saints, their lives and their work almost exclusively from a historical and psychological viewpoint, as though they had no bearing upon the task of theology. This task, however, demands corresponding alterations in method: rather than consider the psychological unfolding from below, it should work out a sort of supernatural phenomenology of their mission from aboveAlong these same lines, Christoph Cardinal Schönborn (in whose book, God Sent His Son, I recently re-encountered this insight of Balthasar's), observed: "The saints are theologians on the basis of their lives and their mission." Perhaps the strongest argument in favor of Divine Mercy Sunday is that Bl. John Paul II worked out the "supernatural phenomenology" of St. Faustina's mission.
St. Faustina's mission was to reveal more of the unfathomable mystery of Divine Mercy. Her mission, which, as she predicted, went through some ups and downs, even being suppressed for a number of years (her journal was placed on the Index Librorum Prohibitorum from 1959 until the Index was abolished in 1966), culminated on 30 April 2000, not with her canonization, but when Blessed Pope John Paul II, her fellow Pole, who unexpectedly came to occupy the Chair of Peter, declared the Second Sunday of Easter to also be Divine Mercy Sunday. Our Blessed Lord communicated this to St. Faustina: "I have opened my Heart as a living fountain of mercy. Let all souls draw life from it. Let them approach this sea of mercy with great trust. Sinners will attain justification, and the just will be confirmed in good. Whoever places his trust in My mercy will be filled with My divine peace at the hour of death." It is a classic Catholic both/and, not a false either/or.
The Church's magisterium has always recognized exactly what Balthasar insisted, which is why Sts. Teresa of Ávila, John of the Cross, Catherine of Siena, Thérèse of Lisieux, and most recently (named in 2012 by Pope Benedict XVI) Hildegard of Bingen, are all Doctors of the Church. "Hence," Cardinal Schönborn commented further, "it is of great importance for theology to be ready to listen to the saints and to learn from them," from what Balthasar termed the saints' "dogmatics of experience." This goes perhaps double for liturgists, pointing us to the overly used (even unto the point of abuse- like St. Augustine's "Our hearts are restless...) lex orandi, lex credendi.
Oh, by the way, tomorrow we observe the Solemnity of the Annunciation of the Lord because to have done so on the Monday of Holy Week, or during the Octave, would have apparently blown our minds. We move this observance, which normally happens nine months to the day prior to Christmas Day, even though it means throwing our liturgical arithmetic off (sorta like celebrating the Ascension on the Seventh Sunday of Easter, instead of forty days after Easter, or ending Christmas on the Feast of the Baptism of the Lord, or making Epiphany a movable observance, being the second Sunday after Christmas, instead of always on 6 January, thus marking the 12 days of Christmas). I find the Paschal Mystery mind blowing in any case.