The Divine Mercy Novena runs from Good Friday to the Vigil of Divine Mercy Sunday (Sunday after Easter).
Jesus asked that the Feast of the Divine Mercy be preceded by a Novena to the Divine Mercy which would begin on Good Friday. He gave St. Faustina an intention to pray for on each day of the Novena, saving for the last day the most difficult intention of all, the lukewarm and indifferent of whom He said: "These souls cause Me more suffering than any others; it was from such souls that My soul felt the most revulsion in the Garden of Olives. It was on their account that I said: 'My Father, if it is possible, let this cup pass Me by.' The last hope of salvation for them is to flee to My Mercy."
In her diary, St. Faustina wrote that Jesus told her: "On each day of the novena you will bring to My heart a different group of souls and you will immerse them in this ocean of My mercy ... On each day you will beg My Father, on the strength of My passion, for the graces for these souls."
For more information about the Divine Mercy devotion, including information about the Divine Mercy Sunday indulgence — and instructions on saying the Divine Mercy Chaplet — go to http://www.EWTN.com/devotionals/mercy