On September 13, 1935, St. Faustina recorded in her diary that she had a vision of Jesus during which he asked her to pray the chaplet and instruct others to do so. According to her diary, the chaplet’s prayers for mercy are intended to obtain mercy, to trust in Christ’s mercy and to show mercy to others. St. Faustina wrote that Jesus promised that all who recite this chaplet at the hour of death or in the presence of the dying will receive great mercy.
Traditionally, the chaplet may be said at any time, but it is said especially on Divine Mercy Sunday (the Sunday after Easter) and Fridays at 3:00 PM (the hour Jesus died by crucifixion, called the Hour of Mercy). As a novena, it is usually said each of the nine days from Good Friday to Divine Mercy Sunday. St. John Paul II was instrumental in extending the devotion to the Divine Mercy worldwide and promoted it in a Papal Blessing in 2001.
For more information about the Chaplet of the Divine Mercy and how to pray it, visit the USCCB page.