Christ appeared to a Polish nun, Sr. Maria Faustina Kowalska, in a series of visions that took place over almost eight years. Among those visions, Christ revealed the Divine Mercy Novena, which He asked Sister Faustina to pray for nine days, beginning on Good Friday. That meant that the novena ended on the Saturday after Easter — the eve of the Octave of Easter. Thus, since novenas are commonly prayed in advance of a feast, the Feast of Divine Mercy — Divine Mercy Sunday — was born.
A plenary indulgence (the forgiveness of all temporal punishment resulting from sins that have already been confessed) is granted on the Feast of Divine Mercy if to all the faithful who go to Confession, receive Holy Communion, pray for the intentions of the Holy Father, and "in any church or chapel, in a spirit that is completely detached from the affection for a sin, even a venial sin, take part in the prayers and devotions held in honour of Divine Mercy, or who, in the presence of the Blessed Sacrament exposed or reserved in the tabernacle, recite the Our Father and the Creed, adding a devout prayer to the merciful Lord Jesus (e.g. 'Merciful Jesus, I trust in you!')."
A partial indulgence (the remission of some temporal punishment from sin) is granted to the faithful "who, at least with a contrite heart, pray to the merciful Lord Jesus a legitimately approved invocation."
(Source: ThoughtCo. "Divine Mercy Sunday." Learn Religions, Aug. 25, 2020, learnreligions.com/divine-mercy-sunday-542469.)
Jesus disclosed to Sr. Faustina the following intentions to pray for on each day of the Novena.
DAY 1 (Good Friday) - All mankind, especially sinners
DAY 2 (Holy Saturday ) - The souls of priests and religious
DAY 3 (Easter Sunday) - All devout and faithful souls
DAY 4 (Easter Monday) - Those who do not believe in Jesus & those who do not know Him
DAY 5 (Easter Tuesday) - The souls of separated brethren
DAY 6 (Easter Wednesday) - The meek and humble souls and the souls of children
DAY 7 (Easter Thursday) - The souls who especially venerate and glorify Jesus' mercy
DAY 8 (Easter Friday) - The souls who are detained in purgatory;
DAY 9 (Easter Saturday) - The souls who have become lukewarm.
The Chaplet of Divine Mercy should be offered each day for the day's intentions.
Click the link for the Divine Mercy Novena https://www.thedivinemercy.org/message/devotions/novena/
Click the link on how to pray the Divine Mercy Chaplet .https://www.thedivinemercy.org/message/devotions/pray-the-chaplet