In a series of revelations to St. Maria Faustina Kowalska in the 1930s, our Lord called for a special feast day to be celebrated on the Sunday after Easter. Today, we know that feast as Divine Mercy Sunday, named by Pope St. John Paul II at the canonization of St. Faustina on April 30, 2000. 

The Lord expressed His will with regard to this feast in His very first revelation to St. Faustina. The most comprehensive revelation can be found in her Diary entry 699:

My daughter, tell the whole world about My inconceivable mercy. I desire that the Feast of Mercy be a refuge and a 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 are opened all the divine floodgates through which graces flow. 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 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 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.

In all, St. Faustina recorded 14 revelations from Jesus concerning His desire for this feast. 

Nevertheless, Divine Mercy Sunday is NOT a feast based solely on St. Faustina's revelations. Indeed, it is not primarily about St. Faustina — nor is it altogether a new feast. The Second Sunday of Easter was already a solemnity as the Octave Day of Easter[1]. The title "Divine Mercy Sunday" does, however, highlight the meaning of the day. 

How Divine Mercy Came to Be - Explaining the Faith