Sunday, April 27, is Divine Mercy Sunday. The celebration was first established on April 30, 2000, the day of the canonization of Sr. Faustina, the apostle of Divine Mercy. This year, it is also the canonization of soon-to-be-St. John Paul II, who declared the Second Sunday of Easter to be Divine Mercy Sunday.
One quick look at the Gospel for that Sunday gives us the reason. Perhaps it was not St. John Paul II that gave us this feast, but Jesus Himself gave it to us on Easter Sunday night.