The history of the Divine Mercy could be traced back from the beginning of the World as written in the Book of Genesis. God, in his merciful love would like to share it to mankind. However, here we speak of the history of the Divine Mercy as a devotion of the faithful.
First, it was a private devotion started by Sr. Faustina (Helen Kowalska) of the Congregation of Sisters of Our Lady of Mercy in Cracow, Poland between the 1931-1938. In her vision she received the an instruction from our Lord Jesus Christ who instructed her in the Divine Mercy devotion, and gave her visions of Heaven and Hell with the aim of saving as many souls as possible. Our Lord instructed St Faustina to have an image made of Him, with red and white rays eminating from His Sacred Heart, and the words "Jesus, I trust in You" engraved below. For a detailed account you can go to http://www.thedivinemercy.org/message/history/.
It was through our late beloved Pope John Paul II who introduced to the Christian Faithful to the devotion to Jesus called "Divine Mercy" in the Year of Our Lord 2000, which coincides with the celebration of the great Jubillee.
When Pope John Paul canonized Sr. Faustina (making her St. Faustina), he also, on the same day, surprised the entire world by establishing Divine Mercy Sunday (the feast day associated with the message) as a feast day for the entire Church. The feast day falls on the Second Sunday of the Easter season. On that day, Pope John Paul II declared, "This is the happiest day of my life."More on http://www.thedivinemercy.org/message/johnpaul/index.php
In a proclaimation that year, the Congregation for Divine Worship and the Discipline of the Sacraments declared the Sunday following Easter Sunday as "Divine Mercy Sunday", and in 2002, the Holy See announced an Indulgence would be associated with this devotion. This was the highest indorsement to a private revelation the Church can give, as private revelations are not binding on the Faithful.As preparation for the Feast of the Divine Mercy this Sunday. I am posting these videos. This is the Chaplet in Songs.