An ancient chant, taken from Psalm 118:24. In the modern usage, this text is used in the Divine Office and for the Gospel Alleluia verse for all 8 days of the Easter octave, today through Divine Mercy Sunday. In Catholic tradition, Easter is too big a deal to fit into just one day, so the celebration of the day of Resurrections is extended over 8 days, and then a season of 40 days until the Ascension to celebrate the Risen Christ with us.
This is the day which the Lord hath made:
let us be glad and rejoice therein.
verse for Easter Sunday:
Give praise to the Lord, for he is good:
for his mercy endureth for ever. (Psalm 118:1)
[Christ our Passover was sacrificed for us.]
verse for Easter Monday:
Let Israel now say, that he is good:
that his mercy endureth for ever. (Psalm 118:2)
verse for Easter Tuesday:
Let them say so that have been redeemed by the Lord,
whom he hath redeemed from the hand of the enemy:
and gathered out of the countries.
Every great composer in the West set this to music for centuries, so, in addition to the epic and wonderful chant setting above, we have any number of other glorious versions:
Bach set this, because of course he did:
Happy, Holy, and Blessed Easter! He is truly risen!