This novena is traditionally prayed every year leading up to Divine Mercy Sunday. Since that’s always the first Sunday after Easter, the first day of the novena is always Good Friday (seems fitting). That said, the Divine Mercy Novena can, of course, be prayed any time of the year.
Jesus revealed this novena to St. Faustina, as recorded in her Diary:
I desire that during these nine days you bring souls to the fount of My mercy, that they may draw there from strength and refreshment and whatever graces they need in the hardships of