Every day this week is a first class feast day, when not even Saint Francis of Paola, Saint Isidore or Saint Vincent Ferrer are commemorated at Mass. The dismissal, only for these seven days, is "Ite Missa Est, Alleluia, Alleluia."
The glorious Easter sequence is said by the celebrant, and ideally sung by a schola at High Mass, every day between Easter Sunday and Easter Saturday before the Gospel. The composer of the sequence is not known for sure, but credit is often given to Wipo of Burgundy, chaplain of the German Emperor Conrad II in the 11th century; Notker Balbulus in the 10th century; and/or Adam of Saint Victor in the 13th century.
Interestingly, the Tridentine missal (1570) omitted the sixth verse of the sequence, which was deemed offensive to Jews. This is why it sort of jumps into the "Scimus Christum" verse toward the end. The same Council of Trent era reform added the "Amen" and "Alleluia" at the end of the sequence.
Take a moment this Easter week to listen to the Benedictine monks of Fontgombault sing this beautiful sequence in Gregorian chant. Hopefully you already heard it at your parish on Sunday. If you are in a choir, or thinking about joining or forming a schola, but have not sung the sequence before, the video of the chant is the notation from the Liber Usualis so you may follow along.
