Divine Mercy Sunday is the first Sunday after Easter. That Sunday is the Feast of Mercy.
What our Church is offering this Sunday is called a plenary indulgence, and what this means, in very simple terms, is that - if we go to Confession, and then receive Holy Communion on that day, we can obtain the complete and total forgiveness of all sins and all punishment.
The following paragraphs are from divinemercysunday.com
Jesus told St. Faustina that this Feast of Mercy would be a very special day when "all the divine floodgates through which graces flow are opened". (Diary 699) Our Lord made a great promise to all those souls who would go to Confession and then receive Him in Holy Communion on the Feast of Mercy, on the Sunday after Easter, which is now called Divine Mercy Sunday throughout the Catholic Church.
Jesus promised that "The soul that will go to Confession and receive Holy Communion shall obtain the complete forgiveness of sins and punishment."(Diary 699) He went on to say "I want to grant a complete pardon to the souls that will go to Confession and receive Holy Communion on the Feast of My Mercy." (Diary 1109)
About the feastday "Divine Mercy Sunday", Jesus said "…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 poor sinners. On that day the very depths of My tender mercy are open. I pour out a whole ocean of graces upon the souls who approach the Fount of My Mercy. On that day all the divine floodgates through which graces flow are opened. Let no soul fear to draw near to Me, even though its sins be as scarlet.... Mankind will not have peace until it turns to the Fount of My Mercy". (Diary 699)
Our Lord said "When you go to confession, to this fountain of My mercy, the Blood and Water which came forth from My Heart always flows down upon your soul…" and "Every time you go to confession, immerse yourself entirely in My mercy with great trust, so that I may pour the bounty of My grace upon your soul. When you approach the confessional, know this, that I Myself am waiting there for you. I am only hidden by the priest, but I Myself act in your soul. Here the misery of the soul meets the God of mercy" (1602) Make your confession before Me. The person of the priest is… only a screen. Never analyze what sort of a priest that I am making use of; open your soul in confession to Me, and I will fill it with My light." (1725)
It is required of all Catholics to confess their serious sins at least once every year. If you haven’t yet met this obligation then take advantage of this outstanding opportunity to receive an outpouring of an ocean of graces that Jesus promises on this day. Those who have already confessed their sins should make room for others.
The Church allows for one to go to Confession for up to about 20 days, before or after Divine Mercy Sunday.
