From the website of the US Shrine to Divine Mercy
In her Diary, St. Faustina records a special promise given to her by Jesus. He told her to communicate it to the whole world:
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. I pour out a whole ocean of graces upon those souls who approach the fount of My mercy (699).
In three places in her Diary, St. Faustina records our Lord’s promises of specific, extraordinary graces:
I want to grant a complete pardon to the souls that will go to Confession and receive Holy Communion on the Feast of My mercy (1109).
Whoever approaches the Fountain of Life on this day will be granted complete forgiveness of sins and punishment (300).
The soul that will go to Confession and receive Holy Communion will obtain complete forgiveness of sins and punishment (699).
Or if you want a longer version:
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.
And if for some reason you can’t make it today, remember this when the clock strikes three each day:
At three o’clock, implore My mercy, especially for sinners; and, if only for a brief moment, immerse yourself in My Passion, particularly in My abandonment at the moment of agony. This is the hour of great mercy. In this hour, I will refuse nothing to the soul that makes a request of Me in virtue of My Passion (Diary, 1320).
As often as you hear the clock strike the third hour, immerse yourself completely in My mercy, adoring and glorifying it; invoke its omnipotence for the whole world, and particularly for poor sinners; for at that moment mercy was opened wide for every soul. In this hour you can obtain everything for yourself and for others for the asking; it was the hour of grace for the whole world — mercy triumphed over justice. (1572)
My daughter, try your best to make the Stations of the Cross in this hour, provided that your duties permit it; and if you are not able to make the Stations of the Cross, then at least step into the chapel for a moment and adore, in the Most Blessed Sacrament, My Heart, which is full of mercy; and should you be unable to step into the chapel, immerse yourself in prayer there where you happen to be, if only for a very brief instant. (1572)
Now you might ask: Where does all this come from? Particualrly if you are a protestant. The answer is here:
In 1931, our Lord appeared to St. Faustina in a vision. She saw Jesus clothed in a white garment with His right hand raised in blessing. His left hand was touching His garment in the area of the Heart, from where two large rays came forth, one red and the other pale. She gazed intently at the Lord in silence, her soul filled with awe, but also with great joy. Jesus said to her:
Paint an image according to the pattern you see, with the signature: Jesus, I trust in You. I promise that the soul that will venerate this image will not perish. I also promise victory over [its] enemies already here on earth, especially at the hour of death. I Myself will defend it as My own glory (Diary, 47, 48). I am offering people a vessel with which they are to keep coming for graces to the fountain of mercy. That vessel is this image with the signature: Jesus, I trust in You (327). I desire that this image be venerated, first in your chapel, and [then] throughout the world (47).
At the request of her spiritual director, St. Faustina asked the Lord about the meaning of the rays in the image. She heard these words in reply:
The two rays denote Blood and Water. The pale ray stands for the Water which makes souls righteous. The red ray stands for the Blood which is the life of souls. These two rays issued forth from the depths of My tender mercy when My agonized Heart was opened by a lance on the Cross. Happy is the one who will dwell in their shelter, for the just hand of God shall not lay hold of him (299). By means of this image I shall grant many graces to souls. It is to be a reminder of the demands of My mercy, because even the strongest faith is of no avail without works (742).
An fyi. the numbers are the references to her diary that the Lord ordered her to keep. You can find it at amazon here.
If you ignore everything else I ever write take this to heart. Take advantages of this grace that is offered without price. Or as I like to say: God’s Mercy or God Justice, pick one.