The first Sunday after Easter, the Octave of Easter, is the Feast of the Divine Mercy, Divine Mercy Sunday. To receive the Extraordinary Graces of this Feast, the only condition is to receive Holy Communion worthily on Divine Mercy Sunday (or the Vigil celebration) by making a good confession beforehand and staying in the state of grace and trusting in His Divine Mercy.
The devotion to Jesus as the Divine Mercy is based on the writings of Saint Faustina, a Polish nun who wrote a diary of about 600 pages recording the revelations she received about God’s mercy. On May 5, 2000, five days after the canonization of St. Faustina, the Vatican decreed that the Second Sunday of Easter would henceforth be known as Divine Mercy Sunday.
From the Diary of St. Mary Faustina Kowalska (1905-1938):
My Heart overflows with great mercy for souls, and especially for poor sinners. If only they could understand that I am the best of Fathers to them and that it is for them that the Blood and Water flowed from My Heart as from a fount overflowing with mercy. For them I dwell in the tabernacle as King of Mercy. I desire to bestow My graces upon souls, but they do not want to accept them. You, at least, come to Me as often as possible and take these graces they do not want to accept. In this way you will console My Heart. Oh, how indifferent are souls to so much goodness, to so many proofs of love! My Heart drinks only of the ingratitude and forgetfulness of souls living in the world. They have time for everything, but they have no time to come to Me for graces. (Diary, 367)
The Image of the Divine Mercy originated from a vision St. Faustina had on Feb. 22, 1931.:
In the evening, when I was in my cell, I saw the Lord Jesus clothed in a white garment. One hand [was] raised in the gesture of blessing, the other was touching the garment at the breast. From beneath the garment, slightly drawn aside at the breast, there were emanating two large rays, one red, the other pale. In silence I kept my gaze fixed on the Lord; my soul was struck with awe, but also with great joy. After a while, Jesus said to me, Paint an image according to the pattern you see, with the signature: Jesus, I trust in You. I desire that this image be venerated, first in your chapel, and [then] throughout the world. (Diary, 47) By means of this image I will grant many graces to souls. (Diary, 742).
In addition to the Graces flowing from the Feast of Divine Mercy, Pope St. John Paul the Great attached an additional Plenary Indulgence to its observation. His Encyclical, Dives in Misericordia (Rich in Mercy) was offered on 30 November 1980. According to one biographer, the Pope felt very close to St. Faustina when he began writing.