In the early twentieth century, St. Faustina Kowalska, a Polish nun, received visits from Jesus and had conversations with him. He appeared to her as “the King of Mercy”, wearing a white garment with red and pale blue rays emanating from his heart. He asked her to have this image made into a painting with the words “Jesus, I trust in You” at the bottom, that the faithful might venerate it. He also specifically asked for a feast of Divine Mercy to be established on the first Sunday after Easter.
Pope St. John Paul II had a great devotion to the Sacred Heart of Jesus. In 2000 he canonized St. Faustina and officially designated the second Sunday of Easter as Divine Mercy Sunday. In 2002 he also established a plenary indulgence for the day, in keeping with the promises that Jesus made to St. Faustina about this devotion.
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!");
A perfect prayer for this feast is the Chaplet of Divine Mercy. It is said using a normal rosary, but with different prayers. It is traditionally said at three o’clock in the afternoon and can be completed in about ten minutes.
Resources from: The Catholic All Year Compendium; EWTN