"Tell the whole world about My mercy and My love. The flames of mercy are burning Me. I desire to pour them on human souls. Oh, what pain they cause Me when they do not want to accept them! ... do whatever is within your power to spread devotion to My mercy. I will make up for what you lack. Tell aching mankind to snuggle close to My merciful Heart, and I will fill it with peace. Tell [all people] that I am Love and Mercy itself." (Diary, 1074) - Jesus to St. Faustina
Some priests I've known have said this devotion is out of place in the Octave of Easter, since we just came from Lent and Holy Week's emphasis on the Passion of Christ - Easter week should be celebratory. I don't think the devotion detracts from the joy of Easter at all. I think we can forget that when Jesus appeared to the Disciples, he often reminded them of his Passion and how necessary it was for him to suffer all of those things in order to enter into his glory. Beginning with Moses and the prophets, interpreting to them all that referred to him in the Scriptures.
Likewise, the Second Sunday of Easter is all about the forgiveness of sins, as well as his holy wounds - testimony to his sorrowful Passion. So, it seems to me the devotion is perfectly situated on the evening of the first day of the week - the Second Sunday of Easter.
Jesus, I trust in you. You yourself established and confirmed this Solemnity.