Divine Mercy Sunday is traditionally celebrated on the Second Sunday of Easter (The Octave of Easter). The essence of this special day is to remember Christ himself being revealed to St. Maria Faustina Kowalska and it was Blessed John Paul II that coined this special solemnity when he canonized St. Faustina on April 30, 2000.
John Paul said in his homily at the Canonization that the Image of The Divine Mercy St. Faustina saw, which is to be venerated on Divine Mercy Sunday, represents the Risen Christ bringing mercy to the world. Blessed John Paul II said: "Jesus shows His hands and His side, to the Apostles. He points, that is, to the wounds of the Passion, especially the wound in His Heart, the source from which flows the great wave of mercy poured out on humanity."
When we look at the Mercy Image of Christ, there are two rays, a white ray and a red ray. Jesus himself explained that the two rays 'represent blood and water'. Divine Mercy Sunday, therefore, point us to the merciful love of God that lies behind the whole Paschal Mystery, the whole mystery of the death, burial and resurrection of Christ made present for us in the Eucharist.
Traditionally, 9 days before this special day, which is Good Friday, the Divine Mercy Novena begins. This novena is very special since it is the very first novena in the Catholic faith and was created by Jesus himself. St. Faustina records in her Diary that Jesus instructed her to write down the Novena which she would begin on Good Friday, 1937. Jesus told her that on each day, she would bring a different group of souls to the Fountain of His Mercy. From the Fountain of His Mercy, these souls would be refreshed and granted graces for the hardships of their earthly life and especially for the hour of their death. Jesus promised St. Faustina that He would refuse nothing to any soul whom she brought to the Fountain of His Mercy. He told her that once these souls were refreshed at the Fountain of His Mercy, He would bring them into His Father’s House, at which time St. Faustina would beg the Father, on the strength of the bitter Passion of Jesus, for graces for these souls. St. Faustina responded that she did not know which souls she should bring to the Fountain of Mercy, but Jesus told her that He would tell her on each day. This is why the novena pray is very special.
In June 2002 Blessed John Paul II granted indulgences to Catholics who recite specific prayers on that day, which were then formally decreed by the Apostolic Penitentiary. The priests are also ordered in that day to lead the prayers in honor of Divine Mercy, inform the parishioners about the Divine Mercy, and hear confessions.
St. Faustina, Pray for us!
Jesus I Trust In you!
-- With Excerpts taken from Wikipedia & CatholicOnline.com