In 1935, St. Faustina received a vision of an angel sent by God to chastise a certain city. She began to pray for mercy, but her prayers were powerless. Suddenly she saw the Holy Trinity and felt the power of Jesus’ grace within her. At the same time she found herself pleading with God for mercy with words she heard interiorly:
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; for the sake of His sorrowful Passion, have mercy on us. (Diary, 475)
As she continued saying this inspired prayer, the angel became helpless and could not carry out the deserved punishment (see 474). The next day, as she was entering the chapel, she again heard this interior voice, instructing her how to recite the prayer that our Lord later called “the Chaplet.” This time, after “have mercy on us” were added the words “and on the whole world” (476). From then on, she recited this form of prayer almost constantly, offering it especially for the dying.
In subsequent revelations, the Lord made it clear that the Chaplet was not just for her, but for the whole world. He also attached extraordinary promises to its recitation.
Encourage souls to say the Chaplet which I have given you (1541). Whoever will recite it will receive great mercy at the hour of death (687). 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 (1541). Priests will recommend it to sinners as their last hope 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 (687). I desire to grant unimaginable graces to those souls who trust in My mercy (687). Through the Chaplet you will obtain everything, if what you ask for is compatible with My will. (1731)
Prayed on ordinary rosary beads, The Chaplet of The Divine Mercy is an intercessory prayer that extends the offering of the Eucharist, so it is especially appropriate to use it after having received Holy Communion at Holy Mass. It may be said at any time, but our Lord specifically told St. Faustina to recite it during the nine days before the Feast of Mercy (the first Sunday after Easter). He then added: “By this Novena, [of Chaplets] I will grant every possible grace to souls.” (796)
It is likewise appropriate to pray the Chaplet during the “Hour of Great Mercy” — three o’clock each afternoon (recalling the time of Christ’s death on the cross). In His revelations to St. Faustina, Our Lord asked for a special remembrance of His Passion at that hour.