The term Octave of Easter may refer either to the eight day period (Octave) from Easter Sunday until the Sunday following Easter, inclusive; or it may refer only to that Sunday after Easter, the Octave Day of Easter (sometimes known as Low Sunday).
That Sunday is also known historically as St. Thomas Sunday and Quasimodo Sunday. Since 1970 Low Sunday has been officially known as the Second Sunday of Easter (referring to the Easter season) in the Roman Catholic Church. On April 30, 2000, it was also designated as Divine Mercy Sunday by Pope John Paul II. Among Eastern Christians the day is known as Thomas Sunday. (From: Wikipedia, license: CCA-SA)