The Octave, or eighth day, of Easter has always been considered special by Christians. Christ, after His Resurrection,
revealed Himself to His disciples, but Saint Thomas wasn't with them. He declared that he would never believe that Christ had risen from the dead until he could see Him in the flesh and probe Christ's wounds with his own hands. This earned him the name "Doubting Thomas."
A week after Christ rose from the dead, He appeared once again to His disciples, and this time Thomas was there. His doubt was vanquished, and he professed His belief in Christ.
Nineteen centuries later, 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.
Indulgences for Divine Mercy Sunday:
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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!')."
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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."
ThoughtCo. "Divine Mercy Sunday." Learn Religions, Aug. 25, 2020, learnreligions.com/divine-mercy-sunday-542469.