Our Lord asked Blessed Faustina to pray and work towards establishing a Feast of The Divine Mercy on the Sunday after Easter. He told her:
On that day, the very depths of My tender mercy are open. I pour out a whole ocean of graces upon 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 (Diary of Blessed Faustina, 699).
It is a day that would celebrate the paschal mystery with a focus on God's covenant of mercy. It would be a day of complete forgiveness and pardon, like the day of atonement in the Old Testament (see Lv 16) -- all our sins and the punishment due to them would be atoned for.
We can already celebrate this "Mercy Sunday" by going to Confession (preferably before that Sunday) and by receiving Communion on that day. We can honor the mercy of the Lord by venerating the Image of The Divine Mercy, and by our prayers and works of mercy.
Jesus appeared to Blessed Faustina with rays of red and pale light streaming from the area around His heart. His right hand was raised in blessing, recalling the scene of Easter Sunday night (see Jn 20: 19-23).
He asked Blessed Faustina to have this vision painted signed with the words, "Jesus, I trust in You!"
He revealed this image to remind people to trust in His mercy, and to come to Him for mercy:
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." (Diary, 327).
Jesus explained that the rays represented the blood and water which flowed from His pierced side, and He taught Blessed Faustina the prayer:
O Blood and Water, which gushed forth from the Heart of Jesus as a fount of mercy for us, I trust in You (Diary, 84).
Our Lord taught Blessed Faustina a prayer for mercy that she was to pray "unceasingly": The Chaplet of The Divine Mercy. He promised:
Encourage souls to say the Chaplet which I have given you ... Whoever will recite it will receive great mercy at the hour of death ... When they say this chaplet in the presence of the dying, I will stand between my Father and the dying person, not as the Just Judge but as the Merciful Savior ... Priests will recommend it to sinners as their last home of salvation. Even if there were a sinner most hardened, if he were to recite this chaplet only once, he would receive grace from my infinite mercy. I desire to grant unimaginable graces to those souls who trust in My mercy ... Through the Chaplet you will obtain everything, if what you ask for is compatible with My will (Diary, 687, 1541, 1731).
We can pray this chaplet, using ordinary rosary beads of five decades. We begin with the Our Father, the Hail Mary and the Apostles Creed.
Then on the large beads we pray:
Eternal Father, I offer you the Body and Blood, Soul and Divinity of Your Dearly Beloved Son, Our Lord, Jesus Christ, in atonement for our sins and those of the whole world.
On the small beads we pray:
For the sake of His sorrowful Passion, have mercy on us and on the whole world.
And at the end, we pray three times:
Holy God, Holy Mighty One, Holy Immortal One, have mercy on us and on the whole world (Diary, 476).
In preparation for the Feast of The Divine Mercy, the Lord asked Blessed Faustina to make a novena of prayer from Good Friday to the following Saturday.
For each of the nine days, our Lord gave Blessed Faustina a different intention:
1st. day - all mankind, especially sinners;
2nd. day - the souls of priests and religious;
3rd. day - all devout and faithful souls;
4th. day - those who do not believe in God and those
who do not yet know Jesus;
5th. day - the souls who have separated themselves from
the Church;
6th. day - the meek and humble souls and the souls of little children;
7th. day - the souls who especially venerate and glorify His mercy;
8th. day - the souls detained
in purgatory; and
9th. day - souls who have become lukewarm.
I desire that during these nine days you bring souls to the fountain of My mercy, that they may draw therefrom strength and refreshment and whatever grace they have need of in the hardships of life, and especially at the hour of death (Diary, 1209).
We, too, can make a novena of prayer for these intentions and others,
especially by praying the Chaplet of The Divine Mercy. The novena can
be prayed any time during the year.
Also, we could pray the following:
I fly to Your Mercy, Compassionate God, Who alone are good.
Although my misery is great and my offenses are many,
I trust in Your Mercy, because You are the God of Mercy,
and it has never been heard of in all ages, nor do Heaven or Earth remember,
that a soul trusting in Your Mercy has been disappointed.
(State your intentions)
Jesus, Friend of a lonely heart,
You are my haven,
You are my peace,
You are my salvation,
You are my serenity in moments of struggle and amidst an ocean of doubts.
Amen.
In His revelations to Blessed Faustina, Jesus asked for special, daily remembrance at three o'clock PM, the very hour He died for us on the cross:
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 for the whole world. I will allow you to enter into My mortal sorrow. In this hour, I will refuse nothing to the soul that makes a request of Me in virtue of My Passion (Diary, 1320).
At 3:00 o'clock PM we can pray:
O Blood and Water, which gushed forth from the Heart of Jesus as a fountain of mercy for us, I trust in You (Diary, 84).