The Responsorial Psalm
61. After the first reading comes the responsorial Psalm, which is an integral part of the Liturgy of the Word and holds great liturgical and pastoral importance, because it fosters meditation on the word of God.
The responsorial Psalm should correspond to each reading and should, as a rule, be taken from the Lectionary.
It is preferable that the responsorial Psalm be sung, at least as far as the people's response is concerned. Hence, the psalmist, or the cantor of the Psalm, sings the verses of the Psalm from the ambo or another suitable place. The entire congregation remains seated and listens but, as a rule, takes part by singing the response, except when the Psalm is sung straight through without a response. In order, however, that the people may be able to sing the Psalm response more readily, texts of some responses and Psalms have been chosen for the various seasons of the year or for the various categories of Saints. These may be used in place of the text corresponding to the reading whenever the Psalm is sung. If the Psalm cannot be sung, then it should be recited in such a way that it is particularly suited to fostering meditation on the word of God.
In the dioceses of the United States of America, the following may also be sung in place of the Psalm assigned in the Lectionary for Mass: either the proper or seasonal antiphon and Psalm from the Lectionary, as found either in the Roman Gradual or Simple Gradual or in another musical setting; or an antiphon and Psalm from another collection of the psalms and antiphons, including psalms arranged in metrical form, providing that they have been approved by the United States Conference of Catholic Bishops or the Diocesan Bishop. Songs or hymns may not be used in place of the responsorial Psalm.
62. After the reading that immediately precedes the Gospel, the Alleluia or another chant indicated by the rubrics is sung, as required by the liturgical season. An acclamation of this kind constitutes a rite or act in itself, by which the assembly of the faithful welcomes and greets the Lord who is about to speak to them in the Gospel and professes their faith by means of the chant. It is sung by all while standing and is led by the choir or a cantor, being repeated if this is appropriate. The verse, however, is sung either by the choir or by the cantor.
The Alleluia is sung in every season other than Lent. The verses are taken from the Lectionary or the Graduale.
During Lent, in place of the Alleluia, the verse before the Gospel is sung, as indicated in the Lectionary. It is also permissible to sing another psalm or tract, as found in the Graduale.
63. When there is only one reading before the Gospel,
During a season when the Alleluia is to be said, either the Alleluia Psalm or the responsorial Psalm followed by the Alleluia with its verse may be used;
During the season when the Alleluia is not to be said, either the psalm and the verse before the Gospel or the psalm alone may be used;
The Alleluia or verse before the Gospel may be omitted if they are not sung.
64. The Sequence, which is optional except on Easter Sunday and on Pentecost Day, is sung before the Alleluia.