During the course of Jesus' revelations to St. Faustina He asked that a feast day be dedicated to the Divine Mercy and that this feast be celebrated on the Sunday after Easter.

"Whoever approaches the Fountain of Life on this day will be granted complete forgiveness of sins and punishment." -- Diary, 300

"This Feast emerged from the very depths of My mercy, and it is confirmed in the vast depths of my tender mercies." -- Diary, 420

"Tell the whole world about My inconceivable mercy. I desire that the Feast of Mercy be a refuge and shelter for all souls, and especially for poor sinners. On that day the very depths of My tender mercy are open. I pour out a whole ocean of graces upon those souls who approach the fount of My mercy. The soul that will go to Confession and receive Holy Communion shall obtain complete forgiveness of sins and punishment. On that day all the divine floodgates through which grace flow are opened. Let no soul fear to draw near to Me, even though its sins be as scarlet. My mercy is so great that no mind, be it of man or of angel, will be able to fathom it throughout all eternity. Everything that exists has come forth from the very depths of My most tender mercy. Every soul in its relation to Me will contemplate My love and mercy throughout eternity. The Feast of Mercy emerged from the My very depths of tenderness. It is My desire that it be solemnly celebrated on the first Sunday after Easter. Mankind will not have peace until it turns to the Fount of My Mercy. -- Diary, 699