In a series of revelations to St. Maria Faustina Kowalska in the 1930s, our Lord called for a special feast day to be celebrated on the Sunday after Easter.  Today, we know that feast as Divine Mercy Sunday, named by Pope St. John Paul II at the canonization of St. Faustina on April 30, 2000. 

The Lord expressed His will with regard to this feast in His very first revelation to St. Faustina. The most comprehensive revelation can be found in her Diary entry 699:

My daughter, tell the whole world about My inconceivable mercy. I desire that the Feast of Mercy be a refuge and a 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 are opened all the divine floodgates through which graces flow. 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 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.

In all, St. Faustina recorded 14 revelations from Jesus concerning His desire for this feast. 

Nevertheless, Divine Mercy Sunday is NOT a feast based solely on St. Faustina's revelations. Indeed, it is not primarily about St. Faustina — nor is it altogether a new feast. The Second Sunday of Easter was already a solemnity as the Octave Day of Easter. The title "Divine Mercy Sunday" does, however, highlight the meaning of the day. 

taken from https://www.thedivinemercy.org

Receive the Sacrament of Reconciliation so that the soul is in the state of grace.  (Confession does not need to be made on that date but before the feast).

Receive Holy Communion on the Feast of Divine Mercy.

Venerate the Image of Jesus, the Divine Mercy.

Be merciful to others through our actions, words, and prayers.

The Nine-Day Novena, beginning on Good Friday, is also a beautiful optional preparation.
 

First Day"Today bring to me all mankind, especially all sinners, and immerse them in the ocean of my mercy. In this way you will console me in the bitter grief into which the loss of souls plunges me." Most merciful Jesus, whose very nature it is to have compassion on us to forgive us, do not look upon our sins, but upon our trust which we place in Your infinite goodness. Receive us all into the abode of Your Most Compassionate Heart, and never let us escape from it. We beg this of You by Your love which unites You to the Father and the Holy Spirit. Eternal Father, turn Your Merciful gaze upon all mankind and especially upon poor sinners, all enfolded in the Most Compassionate Heart of Jesus. For the sake of His Sorrowful Passion show us Your Mercy, that we may praise the infinite power of Your Mercy, forever and forever. Amen."
Second Day"Today bring to me the souls of Priests and Religious and immerse them in my unfathomable mercy. It was they who gave me strength to endure my bitter passion. Through them, as though channels, my mercy flows out upon mankind." Most Merciful Jesus, from whom comes all that is good, increase Your grace in men and women consecrated to Your service that they perform worthy works of mercy, and that all who see them may glorify the Father of Mercy who is in Heaven. Eternal Father, turn Your merciful gaze upon the company of chosen ones in Your vineyard - upon the souls of Priests and Religious - and endow them with the strength of Your Blessing. For the Love of the Heart of Your Son in which they are enfolded, impart to them Your Power and Light, that they may be able to guide others in the way of salvation, and with one voice sing praise to Your boundless Mercy for ages without end. Amen
Third Day "Today bring to me all devout and faithful souls and immerse them in the ocean of my mercy. These souls brought me consolation on the Way of the Cross. They were that drop of consolation in the midst of an ocean of bitterness." Most Merciful Jesus, from the treasury of Your Mercy, You impart Your graces in great abundance to each and all. Receive us into the abode of Your Most Compassionate Heart and never let us escape from it. We beg this grace of You by that most wondrous love for the heavenly Father with which Your Heart burns so fiercely. Eternal Father, turn Your merciful gaze upon faithful souls, as upon the inheritance of Your Son. For the sake of His Sorrowful Passion grant them Your blessing and surround them with Your constant protection. Thus may they never fail in love or lose the treasure of the Holy Faith, but rather, with all the hosts of Angels and Saints, may they glorify Your boundless Mercy for endless ages. Amen
Fourth Day"Today bring to me those who do not believe in God and those who do not yet know me. I was thinking also of them during my bitter passion, and their future zeal comforted my heart. Immerse them in the ocean of my mercy." Most Compassionate Jesus, You are the Light of the whole world. Receive into the abode of Your Most Compassionate Heart the souls of those who as yet do not believe in You or do not know You. Let the rays of Your Grace enlighten them that they, too, together with us, may extol Your wonderful Mercy, and do not let them escape from the abode which is Your Compassionate Heart. Eternal Father, turn Your merciful gaze upon the souls who do not believe in Your Son, and of those who as yet do not know You, but who are enclosed in the Most Compassionate Heart of Jesus. Draw them to the light of the Gospel. These souls do not know what great happiness it is to love You. Grant that they, too, may extol the generosity of Your Mercy for endless ages. Amen
Fifth Day "Today bring to me the souls who have separated from my Church and immerse them in the ocean of my mercy. During my bitter passion they tore at my body and Heart, that is, my Church. As they return to unity with the Church, my wounds heal, and in this way they alleviate my passion." Most Merciful Jesus, Goodness Itself, You do not refuse light to those who seek it of You. Receive into the abode of Your Most Compassionate Heart the souls of those who have separated from Your Church. Draw them by Your Light into the unity of the Church, and do not let them escape from the abode of Your Most Compassionate Heart; but bring it about that they, too, come to glorify the generosity of Your Mercy. Eternal Father, turn Your Merciful gaze upon the souls of those who are separated from Your Son's Church, especially those who have squandered Your Blessings and misused Your Graces by obstinately persisting in their errors. Do not look upon their errors, but upon the love of Your Own Son and upon His Bitter Passion, which He underwent for their sake, since they, too, are enclosed in His Most Compassionate Heart. Bring it about that they may glorify Your great Mercy for endless ages. Amen
Sixth Day "Today bring to me the meek and humble souls and the souls of little children and immerse them in my mercy. These souls most closely resemble my Heart. They strengthened me during my bitter agony. I saw them as earthly angels, who will keep vigil at my altars. I pour out upon them whole torrents of grace. Only the humble souls is capable of receiving my grace. I favor humble souls with my confidence." Most Merciful Jesus, You Yourself have said "Learn from Me for I am meek and humble of heart". Receive into the abode of Your Most Compassionate Heart all meek and humble souls and the souls of little children. These souls send all Heaven into ecstasy, and they are the Heavenly Father's favorites. They are a sweet-smelling bouquet before the Throne of God; God Himself takes delight in their fragrance. These souls have a permanent abode in Your Most Compassionate Heart, O Jesus, and they unceasingly sing out a hymn of love and mercy. Eternal Father, turn Your Merciful gaze upon meek and humble souls and upon the souls of little children, who are enfolded in the abode which is the Most Compassionate Heart of Jesus. These souls bear the closest resemblance to Your Son. Their fragrance rises from the earth and reaches Your very Throne. Father of Mercy and of all Goodness, I beg You by the love You bear these souls and by the delight You take in them. Bless the whole world, that all souls together may sing out the praises of Your Mercy for endless ages. Amen
Seventh Day "Today bring to me the souls who especially venerate and glorify my mercy and immerse them in my mercy. These souls sorrowed most over my passion and entered most deeply into my spirit. They are living images of my compassionate heart. These souls will shine with a special brightness in the next life. Not one of them will go into the fire of hell. I shall particularly defend each one of them at the hour of death." Most Merciful Jesus, whose Heart is Love Itself, receive into the abode of Your Most Compassionate Heart the souls of those who particularly extol and venerate the greatness of Your Mercy. These souls are mighty with the very power of God Himself. In the midst of all afflictions and adversities they go forward confident of Your Mercy. These souls are united to Jesus and carry all mankind on their shoulders. These souls will not be judged severely, but Your Mercy will embrace them as they depart from this life. Eternal Father, turn Your merciful gaze upon the souls who glorify and venerate Your greatest attribute, that of Your fathomless Mercy, and who are enclosed in the Most Compassionate Heart of Jesus. These souls are a living Gospel; their hands are full of deeds of mercy and their spirits, overflowing with joy, sing a canticle of mercy to You, O Most High! I beg You O God: show them Your Mercy according to the hope and trust they have placed in You. Let there be accomplished in them the Promise of Jesus, who said to them that during their life, but especially at the hour of death, the souls who will venerate this fathomless Mercy of His, He, Himself, will defend as His Glory. Amen
Eighth Day "Today bring to me the souls who are detained in purgatory and immerse them in the abyss of my mercy. Let the torrents of my Blood cool down their scorching flames. All these souls are really loved by me. They are making retribution to my justice. It is in your power to bring them relief. Draw all indulgences from the treasury of my Church and offer them on their behalf. Oh, if you only knew the torments they suffer, you would continually offer for them the alms of the spirit and pay off their debt for my justice." Most Merciful Jesus, You Yourself have said that You desire mercy; so I bring into the abode of Your Most Compassionate Heart the souls in Purgatory, souls who are very dear to You, and yet, who must make retribution to Your justice. May the streams of Blood and Water which gushed forth from Your Heart put out the flames of Purgatory, that there, too, the power of Your mercy may be celebrated. Eternal Father, turn Your merciful gaze upon the souls suffering in Purgatory, who are enfolded in the Most Compassionate Heart of Jesus. I beg You, by the sorrowful Passion of Jesus Your Son, and by all the bitterness with which His most sacred Soul was flooded: Manifest Your mercy to the souls who are under Your just scrutiny. Look upon them in no other way but only through the Wounds of Jesus, Your dearly beloved Son; for we firmly believe that there is no limit to Your goodness and compassion. Amen.
Ninth Day "Today bring to me souls who have become lukewarm and immerse them in the abyss of my mercy. These souls wound my Heart most painfully. My souls suffered the most dreadful loathing in the Garden of Olives because of lukewarm souls. They were the reason I cried out: "Father, take this cup away from me, if it be Your will". For them the last hope of salvation is to run to my mercy." Most Compassionate Jesus, You are Compassion Itself. I bring lukewarm souls into the abode of Your Most Compassionate Heart. In this fire of Your Pure Love let these tepid souls, who, like corpses, filled You with such deep loathing, be once again set aflame. O Most Compassionate Jesus, exercise the omnipotence of Your Mercy and draw them into the very ardor of Your Love; and bestow upon them the gift of Holy Love, for nothing is beyond Your power. Eternal Father, turn Your merciful gaze upon lukewarm souls who are nonetheless enfolded in the Most Compassionate Heart of Jesus. Father of Mercy, I beg You by the bitter passion of Your Son and by His three-hour agony on the Cross, let them too, glorify the Abyss of Your Mercy. Amen