Pope John Paul II, who decreed the universal observance of Divine Mercy Sunday in 2000 and had promoted the devotion for decades before that as archbishop of Krakow in Faustina Kowalska's own home diocese, died on 2 April 2005, the vigil of Divine Mercy Sunday that year, a coincidence Catholic commentators and his own supporters have widely noted and treated as deeply fitting given his personal association with the devotion, and one that Pope Benedict XVI referenced directly when he beatified John Paul II in 2011, a ceremony itself held on Divine Mercy Sunday.
The devotion has since spread substantially through Catholic media, including international broadcasters and publishing houses that produce and distribute the Divine Mercy image, chaplet prayer cards and novena booklets worldwide, so that a devotion originating in the private reported visions of a single Polish nun in the 1930s, initially treated with official suspicion by the Vatican for several decades, is now among the most visibly promoted and widely practised devotions in the entire modern Catholic Church, its spread substantially accelerated by the personal advocacy of the pope who happened to die on its eve.
The feast sits within a wider daily devotion Faustina's Diary also describes: Catholics who practise the Divine Mercy devotion are encouraged to pause at three o'clock each afternoon, the Hour of Mercy, recalling the hour of Christ's death on the cross, to pray the Divine Mercy chaplet, so that the annual Sunday feast this entry describes is, for practising devotees, the high point of a devotion otherwise observed daily throughout the year rather than a once-a-year practice on its own.