Divine Mercy Sunday (also known as the Feast of the Divine Mercy) is a feast day that is observed in the Roman Rite calendar. It is celebrated on the Second Sunday of Easter, which concludes the Octave of Easter. It is originally based on the Catholic devotion to the Divine Mercy that St. Faustina Kowalska reported as part of her visions of Jesus, and is associated with special promises from Jesus and indulgences issued by the Catholic Church.
According to the diary St. Faustina, the feast of Divine Mercy receives the biggest promises of grace from Jesus in the Divine Mercy devotion. A person who goes to sacramental confession (the confession may take place some days before) and receives holy communion on that Sunday, shall obtain the total forgiveness of sins and punishments. This grace is similar to the grace received at baptism and greater than a plenary indulgence. Additionally, the Catholic Church also grants a plenary indulgence (observing the usual rules) with the recitation of some simple prayers.