The hour of the Lord Jesus' death, 3pm, is a privileged time as regards the devotion to the Divine Mercy. At that hour, in spirit we stand beneath Christ’s cross in order to entreat for mercy for us and the whole world for the sake of the merits of His passion. The Lord Jesus said to Sister Faustina: “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” (Diary 1320). Immediately, it needs to be explained that what is meant here is not sixty minutes but the moment of Jesus' agony on the cross, therefore – the prayer at 3pm.
The Lord Jesus did not say what the exact wording of the prayer at 3pm should be, however, He said that we can make the Stations of the Cross, visit the Blessed Sacrament and if there is no time for it – if only for a brief moment – we can unite ourselves with Him, dying on the cross, wherever we happen to be at that time. The subject-matter of the prayer is the mystery of the Lord’s Passion. The prayer said at the Hour of Mercy should meet specific conditions, namely, it should be said at 3 p.m. (when the clock strikes the hour), it should be addressed to Jesus directly and in its entreaties the prayer should refer to the values and merits of His sorrowful passion.
It is frequently written and said that the Chaplet to the Divine Mercy must be said at 3 pm and that the Chaplet said at that time has a special power. It is thought so due to lack of knowledge of the devotion to the Divine Mercy in the forms conveyed by Sister Faustina and maybe due to the need for a ready-made wording of the prayer. The prayer at the Hour of Mercy is a separate form of devotion to the Divine Mercy. Jesus associated a specific promise with it and He also said how it should be said. He never mentioned that the Chaplet to the Divine Mercy should be said at that hour. Certainly, at 3pm we can say the Chaplet, as at any other hour during the day and night, however then we do not say the prayer at the Hour of Mercy (but we say the Chaplet). The Chaplet to the Divine Mercy cannot be the prayer at the Hour of Mercy because it is directed to God the Father (Eternal father, I offer you…), and the prayer at the Hour of Mercy is to be addressed to Jesus directly.
The prayer at the Hour of Mercy when said regularly makes us start having a personal, direct contact with Jesus because it tells us to: meditate upon His Mercy revealed in the Passion, address to Him in a direct manner, like to a very close person, and, it tells us, for the sake of the merits of His Passion, to entreat Him for the graces necessary for us and the world. It is up to us whether at 3pm we want to practice the prayer at the Hour of Mercy or to say the Chaplet to the Divine Mercy. While telling others about the devotion to the Divine Mercy and while practicing it, this should be stated clearly and the Chaplet should not be confused with the Hour of Mercy.