We like being with those whom we love especially when they are suffering. Through the intercession of St Faustina, Jesus asked us to encounter Him in our prayer at 3pm, the hour of His great suffering. Jesus died on the cross and His heart was pierced by the soldier’s word. Blood and water gushed from His side that is in torrents of DivineMercy.
Because Jesus told St Faustina « Every time you hear the clock strike three, plunge yourself entirely in My Mercy, adoring and glorifying it; ask its almighty power for the whole world especially poor sinners ». (Journal § 309)
We can make « the Mercy Hour » by simply stopping for a few seconds (at work, at school, etc.) and thinking of Jesus who suffered His Passion for us and ask Him to shower His Mercy on the world. We can say, for example, St Faustina’s prayer : « O Blood and Water which gushed from the Heart of Jesus, as a Source of Mercy for us, I place my trust in you ! » (Journal § 309)