The Sunday after Easter is Divine Mercy Sunday. Jesus Himself asked for this date to be set aside as Divine Mercy Sunday as part of His revelations to St. Faustina Kowalska, which she has revealed in her diary of Divine Mercy. He also gave to her a novena to Divine Mercy that was to be prayed in preparation for Divine Mercy Sunday. The novena begins on Good Friday. You can pray the novena at any time.
   
Early in the 20th century, Jesus began appearing to Sr. Faustina, revealing Himself in the image of Divine Mercy, and speaking of the mercy that He wished to pour out upon sinners, inviting them to trust in His mercy and to return to Him with all their hearts.
   
You can find the novena at www.thedivinemercy.org. It will explain what you need to do and give you the text of the Novena to be prayed each day.