Concerning the Feast of Mercy Jesus said:

Whoever approaches the Fountain of Life on this day will be granted complete forgiveness of sins and punishment. (Diary 300)

I want the image solemnly blessed on the first Sunday after Easter, and I want it to be venerated publicly so that every soul may know about it. (Diary 341)

This Feast emerged from the very depths of My mercy, and it is confirmed in the vast depths of my tender mercies. (Diary 420)

On one occasion, I heard these words: My daughter, tell the whole world about My Inconceivable mercy. I desire that the Feast of Mercy be a refuge and 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.* [our emphasis] On that day all the divine floodgates through which grace flow are opened. 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 forth 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. (Diary 699)

Yes, the first Sunday after Easter is the Feast of Mercy, but there must also be deeds of mercy, which are to arise out of love for Me. You are to show mercy to our neighbors always and everywhere. You must not shrink from this or try to absolve yourself from it. (Diary 742)

I want to grant complete pardon to the souls that will go to Confession and receive Holy Communion on the Feast of My mercy. (Diary 1109)

As you can see the Lord’s desire for the Feast includes the solemn, public  veneration of the Image of Divine Mercy by the Church, as well as personal acts of veneration and mercy. The great promise for the individual soul is that a devotional act of sacramental penance and Communion will obtain for that soul the plenitude of the divine mercy on the Feast.

*The Cardinal of Krakow, Cardinal Macharski, whose diocese is the center of the spread of the devotion and the sponsor of the Cause of Sr. Faustina, has written that we should use Lent as preparation for the Feast and confess even before Holy Week! So, it is clear that the confessional requirement does not have to be met on the Feast itself. That would be an impossible burden for the clergy if it did. The Communion requirement is easily met that day, however, since it is a day of obligation, being Sunday. We would only need confession again, if received earlier in Lenten or Easter Season, if we were in the state of mortal sin on the Feast.