It is the Second Sunday of Easter.
The Feast of the Lord’s Resurrection is so great that eight of our calendar days are folded into one, great liturgical day! That’s right, we celebrate the Octave of Easter; the preface of the Eucharistic prayer on the “following” Sunday is exactly the same as that for Easter morning, “We praise you with greater joy than ever on this Easter day…” Just as Jesus stood up “on the last day of the feast [of Tabernacles], the great day,” and promised that the living water of the Spirit would flow from his heart to ours; so the Church recognizes the fulfillment of his promise on the last day of its octave. To help us understand the significance of the Easter Octave, Pope John Paul II declared that the Second Sunday of Easter would be known throughout the Church as Divine Mercy Sunday.
When he did so, at the canonization Mass of St. Mary Faustina Kowalska, he invited each of us to “accept the whole message that comes to us from the word of God on this Second Sunday of Easter ... 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 relationship of fraternal solidarity among human beings.” He continued:
"The Church sings. . .as if receiving from Christ’s lips these words of the Psalm. . . 'Give thanks to the Lord for he is good; is steadfast love endures for ever' (Ps.118:1) ... Jesus shows his hands and his side. 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 ... [He] bears the great message of divine mercy and entrusts it to the ministry of the Apostles in the Upper Room: 'Peace be with you. As the Father has sent me, even so I send you ... Receive the Holy Spirit. If you forgive the sins of any, they are forgiven; if you retain the sins of any, they are retained'” (Jn.20:21-23).
As an incentive for the faithful’s special celebration of the day, John Paul also made it an opportunity for gaining a plenary indulgence. His reasons for doing so are quite moving: “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 neighbor, and after they have obtained God's pardon, they in turn might be persuaded to show a prompt pardon to their brothers and sisters.” 
To obtain the indulgence a person must:
a) “take part in the prayers and devotions held in honor of Divine Mercy,” orb) “recite the Our Father and the Creed, adding a devout prayer to the merciful Lord Jesus (e.g. ‘Merciful Jesus, I trust in you!’)”
These are of course in addition to the usual requirements: sacramental Confession, detachment from affection for even venial sin, receiving the Eucharist, and praying for the intentions of the Pope.
Divine Mercy Sunday is now one of the Church’s great treasures, an opportunity for all of us to appropriate the graces won by our Lord’s Passover. Tied as it is to the very heart of the Gospel, it is an unassailable development of the liturgical year. The impetus for John Paul to recognize the value in such a development however, was a matter of private devotion, the Divine Mercy Devotion.