Divine Mercy Sunday is celebrated on the Second Sunday of Easter. It is based on the private revelations of St. Faustina Kowalska, which recommended a particular devotion to the Divine Mercy.
It also has links to the Bible and the readings of this day.
To learn more about St. Faustina, you can CLICK HERE.
In 2000, Pope John Paul II canonized St. Faustina and, during the ceremony, he declared:
It is important then that we accept the whole message that comes to us from the word of God on this Second Sunday of Easter, which from now on throughout the Church will be called "Divine Mercy Sunday".
In the various readings, the liturgy seems to indicate the path of mercy which, while re-establishing the relationship of each person with God, also creates new relations of fraternal solidarity among human beings [Homily, April 30, 2000].
Among other things, it offers a plenary indulgence:
To ensure that the faithful would observe this day with intense devotion, the Supreme Pontiff [John Paul II] himself established that this Sunday be enriched by a plenary indulgence, as will be explained below, so that the faithful might receive in great abundance the gift of the consolation of the Holy Spirit.
In this way, they can foster a growing love for God and for their neighbour, and after they have obtained God's pardon, they in turn might be persuaded to show a prompt pardon to their brothers and sisters. . . .
a plenary indulgence, granted under the usual conditions (sacramental confession, Eucharistic communion and prayer for the intentions of Supreme Pontiff) to the faithful who, on the Second Sunday of Easter or Divine Mercy Sunday, in any church or chapel, in a spirit that is completely detached from the affection for a sin, even a venial sin, take part in the prayers and devotions held in honour of Divine Mercy, or who, in the presence of the Blessed Sacrament exposed or reserved in the tabernacle, recite the Our Father and the Creed, adding a devout prayer to the merciful Lord Jesus (e.g. Merciful Jesus, I trust in you!").
For more information about the plenary indulgence, CLICK HERE.
The Divine Mercy image is a depiction of Jesus based on a vision that St. Faustina had in 1931. There have been a number of paintings made of this image. A contemporary image is shown below.
A basic explanation of the image is:
Jesus is shown in most versions as raising his right hand in blessing, and pointing with his left hand on his chest from which flow forth two rays: one red and one white (translucent).
The depictions often contains the message "Jesus, I trust in You!" (Polish: Jezu ufam Tobie).
The rays streaming out have symbolic meaning: red for the blood of Jesus (which is the Life of Souls), and pale for the water (which justify souls) (from Diary - 299). The whole image is symbolic of charity, forgiveness and love of God, referred to as the "Fountain of Mercy".
According to the diary of St Faustina, the image is based on her 1931 vision of Jesus [source].
(Thanks to National Catholic Register's Jimmy Akin.)