Traditionally this is known as the Second Sunday of Easter on the liturgical calendar. Since 2000, it is known in the Catholic Church as Divine Mercy Sunday. Divine Mercy Sunday is not a feast on its own but is used to close out  Easter as it is 8th and last day of the Easter feast. This is based on the fact that many Jewish observances also last for 8 days. St. Faustina Kowalska also revealed in her revelations that Jesus wanted this 8th day to be part of the Easter feast.