Divine Mercy Sunday is kept on the Second Sunday of Easter, the octave day of the resurrection, an ancient and highly ranked Sunday given a new dedication in the twentieth century. It rests on the private revelations recorded by the Polish nun Maria Faustina Kowalska (1905-1938) in her Diary, in which she reported Christ asking that the Sunday after Easter be kept as a feast of mercy, together with the image of the merciful Christ and the "Divine Mercy" chaplet.
The cause had a complicated history: the devotion was for a time restricted by the Holy See amid doubts about the translations of Faustina's writings, then rehabilitated. Pope John Paul II, himself Polish and a long-time promoter of the devotion, canonised Faustina and decreed the universal observance of Divine Mercy Sunday in 2000. The feast is thus both very old in its liturgical slot and very new in its theme, and it is a clear case of a modern mystic's reported experience being received into the general calendar of the church, a process that in earlier centuries produced feasts such as Corpus Christi.