Our Lord told St. Sr. Faustina:
This Feast emerged from My most tender pity and it is confirmed in the depths of My mercy... I desire that it be celebrated with great solemnity on the first Sunday after Easter... I desire that the Feast of Mercy be a refuge and shelter for all souls, and especially for poor sinners. The very depths of My tender mercy are open on that day. I pour out a whole ocean of graces upon souls who will approach the fount of My mercy.
To the celebration of the Feast, Jesus attached an extraordinary promise:
The soul that will go to Confession and receive Holy Communion [on that day] shall obtain complete forgiveness of sins and punishment... Let no soul fear to draw near to Me, even though its sins be as scarlet.
To observe Divine Mercy Sunday properly, one thing we must do is go to confession, preferably before that Sunday. Think of this confession as the big annual "spring cleaning" time for your soul. Once your inner house is all cleaned up, you will be ready to receive the Lord Himself in Holy Communion and enthrone Him in the center of your heart as your King!
Going to Confession is not the only way we should prepare ourselves for Divine Mercy Sunday. As Cardinal Francis Macharski, then Archbishop of Cracow, Poland, explains in a 1985 pastoral letter, we are not simply called to ask for God's mercy with trust. We are also called to be merciful:
“Our own merciful attitude is likewise a preparation. Without deeds of mercy, our devotion would not be real. For Christ does not only reveal the mercy of God, but at the same time He places before people the demand that they conduct themselves in life with love and mercy. Pope John Paul II states that this requirement constitutes the very heart of the Gospel ethos (Rich in Mercy, 3) — it is the commandment of love and the promise: 'Blessed are the merciful, for they shall obtain mercy' (Mt 5:7). Let it be a mercy that is forgiving and true, and universal, with good words, deeds, and prayer for others!"
Our Lord's words to St. Faustina about this requirement to be merciful are very strong and leave no room for misinterpretation:
“Yes, the first Sunday after Easter is the Feast of Mercy, but there must also be acts of mercy. I demand from you deeds of mercy, which are to arise out of love for Me. You are to show mercy to your neighbors always and everywhere. You must not shrink from this or try to excuse or absolve yourself from it.” [Diary Entry # 742]
Thus, to fittingly observe the Feast of Mercy, we should:
Celebrate the Feast on the Sunday after Easter;
Sincerely repent of all our sins;
Place our complete trust in Jesus;
Go to confession, preferably before that Sunday;
Receive Holy Communion on the day of the Feast;
Venerate the Image of The Divine Mercy;
Be merciful to others, through our actions, words, and prayers on their behalf.