One way the church highlights the preeminence of the Easter season in the liturgical year is by prescribing two special sung texts called sequences to bookend the season on Easter Sunday and Pentecost Sunday.
Historically, the gradual (the psalm between the first reading and the Gospel) ended with an “alleluia” in which the final syllable “ah” was sung over a long series of notes called a melisma. This extension of the gradual became the sequencia. Religious writers began creating alternative texts to sing in place of “ah.” Eventually these developed into completely separate pieces for each liturgical feast, with new melodies and religious poetry. The dramatic text of these musical poems was often sung antiphonally, with verses alternating between two choirs or a soloist and choir.The Easter and Pentecost sequences are sung by the choir, cantor, or assembly after the second reading, before the Alleluia. Click To Tweet
At one point, there were about five thousand sequences in the liturgical calendar! This extravagance was part of the problem progressive solemnity tries to address. Today, only the Easter and Pentecost sequences are required (see GIRM 64), and those for the Body and Blood of Christ and Our Lady of Sorrows are optional.
The Easter and Pentecost sequences are sung by the choir, cantor, or assembly after the second reading, before the Alleluia. The texts, found in the Lectionary, have been set to traditional chants as well as newly composed melodies and adaptations, which are available in most hymnals.