The Acclamation Before the Gospel
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 it in the Gospel and professes its 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 Gradual.
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 Gradual.
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.
The Homily
65. The homily is part of the Liturgy and is strongly recommended,[63] for it is necessary for the nurturing of the Christian life. It should be an exposition of some aspect of the readings from Sacred Scripture or of another text from the Ordinary or from the Proper of the Mass of the day and should take into account both the mystery being celebrated and the particular needs of the listeners.
66. The homily should ordinarily be given by the priest celebrant himself. He may entrust it to a concelebrating priest or occasionally, according to circumstances, to the deacon, but never to a lay person. In particular cases and for a just cause, the homily may even be given by a Bishop or a priest who is present at the celebration but cannot concelebrate.
There is to be a homily on Sundays and holy days of obligation at all Masses that are celebrated with the participation of a congregation; it may not be omitted without a serious reason. It is recommended on other days, especially on the weekdays of Advent, Lent, and the Easter Season, as well as on other festive days and occasions when the people come to church in greater numbers.[66]
After the homily a brief period of silence is appropriately observed.
The Profession of Faith
67. The purpose of the Symbolum or Profession of Faith, or Creed, is that the whole gathered people may respond to the word of God proclaimed in the readings taken from Sacred Scripture and explained in the homily and that they may also call to mind and confess the great mysteries of the faith by reciting the rule of faith in a formula approved for liturgical use, before these mysteries are celebrated in the Eucharist.
68. The Creed is to be sung or said by the priest together with the people on Sundays and solemnities. It may be said also at particular celebrations of a more solemn character.
If it is sung, it is begun by the priest or, if this is appropriate, by a cantor or by the choir. It is sung, however, either by all together or by the people alternating with the choir.
If not sung, it is to be recited by all together or by two parts of the assembly responding one to the other.
From: GENERAL INSTRUCTION OF THE ROMAN MISSAL