The Feast of Mercy is celebrated on the first Sunday after Easter, that is on the Second Sunday of Easter, which is called the Divine Mercy Sunday today. Cardinal Franciszek Macharski was the first one to make it part of the liturgical calendar of the Archdiocese of Cracow (1985) and, later, some Polish bishops instituted the feast in their dioceses. In 1995, at the request of the Polish Episcopal Conference, Holy Father John Paul II instituted the feast in all dioceses in Poland. On the day of Sister Faustina’s canonization, 30th April 2000, the Pope instituted the feast for the whole Church.
The institution of the feast was inspired by a desire of Jesus which was conveyed by Sister Faustina. The Lord Jesus told her: I desire that the first Sunday after Easter be the Feast of Mercy (Diary 299). I desire that the Feast of Mercy be a refuge and shelter for all souls, and especially for poor sinners. On that day the very depths of My tender Mercy are open. I pour out a whole ocean of graces upon those souls who approach the Fount of My Mercy. The soul that will go to Confession and receive Holy Communion shall obtain complete forgiveness of sins and punishment. On that day all the divine floodgates through which graces flow are opened (Diary 699). In many revelations the Lord Jesus did not only define the place of the feast in the liturgical calendar of the Church but also the reason for its institution, the way it should be prepared, celebrated, and He also spoke about the great promises. The greatest of them is the grace of complete forgiveness of sins and punishment. It is connected with Holy Communion received on that day, following a good Confession (without any attachment to the smallest sin), in the spirit of the devotion to the Divine Mercy, that is, trusting God and loving neighbour actively. As the Rev. Professor Ignacy Różycki says, it is a grace greater than plenary indulgence. Plenary indulgence is only a remission of temporal punishment for the sins committed, however, it is never a remission of the sins themselves. Basically, the most special grace is greater than the graces of the 6 sacraments, apart from the sacrament of baptism, since only the sacramental grace of the holy baptism is the forgiveness of all sins and punishment. However, as far as these promises are concerned, Christ associated the remission of sins and punishment with Holy Communion received during the Feast of Mercy, therefore, as regards this, He raised it to the rank of the 'second baptism'. In preparation for this feast we are to recite the novena which consists in saying the Chaplet to the Divine Mercy for 9 days. The novena begins on Good Friday. The Lord Jesus said to Sister Faustina: The Feast of My Mercy has issued forth from My very depths for the consolation of the whole world (Diary 1517).