These days of the week following Easter, and that end on Mercy Sunday, traditionally prolong the feast-day celebrations of Easter and are known as an octave. Octaves are celebrated in the life of the Church because the person, or mystery, commemorated merits a prolonged period of celebration. Easter and its octave take first place among all the octaves of the Church, because they commemorate the central truth of the Catholic faith: that is, that Jesus Christ is alive, risen, and seated at the right hand of God the Father, after having forfeited His life on the Cross for our salvation.
On the second Sunday of Easter, that is eight days after Easter, we venerate and celebrate the feast of Divine Mercy, because it is the source of all that we have commemorated during the week of Easter. According to the keepers of the official shrine of Divine Mercy in Stockbridge, MA, “The Octave Day of Easter, now known as Divine Mercy Sunday, points us to the merciful love of God that lies behind the whole Paschal Mystery ~ the whole mystery of the death, burial and resurrection of Christ made present for us in the Eucharist. In this way, it also sums up the whole Easter Octave. As Pope John Paul II pointed out in his Regina Caeli address on Divine Mercy Sunday, 1995: ‘the whole octave of Easter is like a single day,’ and the Octave Sunday is meant to be the day of ‘thanksgiving for the goodness God has shown to man in the whole Easter mystery'” (Apostles of Divine Mercy, 2002-2011).