From the General Instruction of the Roman Missal
The Mass is made up of two parts: the Liturgy of the Word and the Liturgy of the Eucharist. There are also certain rites that open and conclude the celebration.
THE INTRODUCTORY RITES
The rites preceding the Liturgy of the Word, namely the Entrance, Greeting, Act of Penitence, Kyrie, Gloria, and Collect, have the character of a beginning, introduction, and preparation. Their purpose is to ensure that the faithful who come together as one establish communion and dispose themselves to listen properly to God's word and to celebrate the Eucharist worthily.
After the people have gathered, the Entrance chant begins as the priest enters with the deacon and ministers. The purpose of this chant is to open the celebration, foster the unity of those who have been gathered, introduce their thoughts to the mystery of the liturgical season or festivity, and accompany the procession of the priest and ministers.
When they reach the sanctuary, the priest, the deacon, and the ministers reverence the altar with a profound bow. As an expression of veneration, moreover, the priest and deacon then kiss the altar itself; as the occasion suggests, the priest also incenses the cross and the altar. When the Entrance chant is concluded, the priest stands at the chair and, together with the whole gathering, makes the Sign of the Cross. Then he signifies the presence of the Lord to the community gathered there by means of the Greeting. By this Greeting and the people's response, the mystery of the Church gathered together is made manifest.
The Act of Penitence
Then the priest invites those present to take part in the Act of Penitence, which, after a brief pause for silence, the entire community carries out through a formula of general confession. The rite concludes with the priest's absolution, which, however, lacks the efficacy of the Sacrament of Penance.
The Kyrie Eleison
After the Act of Penitence, the Kyrie is always begun, unless it has already been included as part of the Act of Penitence.
The Gloria is a very ancient and venerable hymn in which the Church, gathered together in the Holy Spirit, glorifies and entreats God the Father and the Lamb.
Next the priest invites the people to pray. All, together with the priest, observe a brief silence so that they may be conscious of the fact that they are in God's presence and may formulate their petitions mentally. Then the priest says the prayer which is customarily known as the Collect and through which the character of the celebration is expressed.
THE LITURGY OF THE WORD
The main part of the Liturgy of the Word is made up of the readings from Sacred Scripture together with the chants occurring between them. The Homily, Profession of Faith, and Prayer of the Faithful, however, develop and conclude this part of the Mass.
The Biblical Readings
In the readings, the table of God's word is prepared for the faithful, and the riches of the Bible are opened to them. The reading of the Gospel is the high point of the Liturgy of the Word. The Liturgy itself teaches that great reverence is to be shown to it by setting it off from the other readings with special marks of honor: whether the minister appointed to proclaim it prepares himself by a blessing or prayer; or the faithful, standing as they listen to it being read, through their acclamations acknowledge and confess Christ present and speaking to them; or the very marks of reverence are given to the Book of the Gospels.
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. 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.
The homily is part of the Liturgy and is strongly recommended, 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. (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.)
The Profession of Faith
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.
The Prayer of the Faithful
In the Prayer of the Faithful, the people respond in a certain way to the word of God which they have welcomed in faith and, exercising the office of their baptismal priesthood, offer prayers to God for the salvation of all.
THE LITURGY OF THE EUCHARIST
At the Last Supper Christ instituted the Paschal Sacrifice and banquet by which the Sacrifice of the Cross is continuously made present in the Church whenever the priest, representing Christ the Lord, carries out what the Lord himself did and handed over to his disciples to be done in his memory. For Christ took the bread and the chalice and gave thanks; he broke the bread and gave it to his disciples, saying, "Take, eat, and drink: this is my Body; this is the cup of my Blood. Do this in memory of me." Accordingly, the Church has arranged the entire celebration of the Liturgy of the Eucharist in parts corresponding to precisely these words and actions of Christ:
1. At the Preparation of the Gifts, the bread and the wine with water are brought to the altar, the same elements that Christ took into his hands.
2. In the Eucharistic Prayer, thanks is given to God for the whole work of salvation, and the offerings become the Body and Blood of Christ.
3. Through the Fraction and through Communion, the faithful, though they are many, receive from the one bread the Lord's Body and from the one chalice the Lord's Blood in the same way the Apostles received them from Christ's own hands.
The Preparation of the Gifts
At the beginning of the Liturgy of the Eucharist the gifts, which will become Christ's Body and Blood, are brought to the altar. First, the altar, the Lord's table, which is the center of the whole Liturgy of the Eucharist, is prepared by placing on it the corporal, purificator, Missal, and chalice (unless the chalice is prepared at the credence table). The offerings are then brought forward. It is praiseworthy for the bread and wine to be presented by the faithful. They are then accepted at an appropriate place by the priest or the deacon and carried to the altar. Even though the faithful no longer bring from their own possessions the bread and wine intended for the liturgy as in the past, nevertheless the rite of carrying up the offerings still retains its force and its spiritual significance.
Once the offerings have been placed on the altar and the accompanying rites completed, the invitation to pray with the priest and the prayer over the offerings conclude the preparation of the gifts and prepare for the Eucharistic Prayer.
The Eucharistic Prayer
Now the center and summit of the entire celebration begins: namely, the Eucharistic Prayer, that is, the prayer of thanksgiving and sanctification. The Eucharistic Prayer demands that all listen to it with reverence and in silence.
The chief elements making up the Eucharistic Prayer may be distinguished in this way:
a. Thanksgiving (expressed especially in the Preface): In which the priest, in the name of the entire holy people, glorifies God the Father and gives thanks for the whole work of salvation or for some special aspect of it that corresponds to the day, festivity, or season.
b. Acclamation: In which the whole congregation, joining with the heavenly powers, sings the Sanctus. This acclamation, which is part of the Eucharistic Prayer itself, is sung or said by all the people with the priest.
c. Epiclesis: In which, by means of particular invocations, the Church implores the power of the Holy Spirit that the gifts offered by human hands be consecrated, that is, become Christ's Body and Blood, and that the spotless Victim to be received in Communion be for the salvation of those who will partake of it.
d. Institution narrative and consecration: In which, by means of words and actions of Christ, the Sacrifice is carried out which Christ himself instituted at the Last Supper, when he offered his Body and Blood under the species of bread and wine, gave them to his Apostles to eat and drink, and left them the command to perpetuate this same mystery.
e. Anamnesis: In which the Church, fulfilling the command that she received from Christ the Lord through the Apostles, keeps the memorial of Christ, recalling especially his blessed Passion, glorious Resurrection, and Ascension into heaven.
f. Offering: By which, in this very memorial, the Church—and in particular the Church here and now gathered—offers in the Holy Spirit the spotless Victim to the Father. The Church's intention, however, is that the faithful not only offer this spotless Victim but also learn to offer themselves, and so day by day to be consummated, through Christ the Mediator, into unity with God and with each other, so that at last God may be all in all.
g. Intercessions: By which expression is given to the fact that the Eucharist is celebrated in communion with the entire Church, of heaven as well as of earth, and that the offering is made for her and for all her members, living and dead, who have been called to participate in the redemption and the salvation purchased by Christ's Body and Blood.
h. Final doxology: By which the glorification of God is expressed and is confirmed and concluded by the people's acclamation, Amen.
The Communion Rite
Since the Eucharistic Celebration is the Paschal Banquet, it is desirable that in keeping with the Lord's command, his Body and Blood should be received by the faithful who are properly disposed as spiritual food. This is the sense of the fraction and the other preparatory rites by which the faithful are led directly to Communion.
The Lord's Prayer
In the Lord's Prayer a petition is made for daily food, which for Christians means preeminently the eucharistic bread, and also for purification from sin, so that what is holy may, in fact, be given to those who are holy.
The Rite of Peace
The Rite of Peace follows, by which the Church asks for peace and unity for herself and for the whole human family, and the faithful express to each other their ecclesial communion and mutual charity before communicating in the Sacrament.
The priest breaks the Eucharistic Bread, assisted, if the case calls for it, by the deacon or a concelebrant. Christ's gesture of breaking bread at the Last Supper, which gave the entire Eucharistic Action its name in apostolic times, signifies that the many faithful are made one body (1 Cor 10:17) by receiving Communion from the one Bread of Life which is Christ, who died and rose for the salvation of the world.
The priest prepares himself by a prayer, said quietly, that he may fruitfully receive Christ's Body and Blood. The faithful do the same, praying silently. The priest next shows the faithful the Eucharistic Bread, holding it above the paten or above the chalice, and invites them to the banquet of Christ. Along with the faithful, he then makes an act of humility using the prescribed words taken from the Gospels.
It is most desirable that the faithful, just as the priest himself is bound to do, receive the Lord's Body from hosts consecrated at the same Mass and that, in the instances when it is permitted, they partake of the chalice, so that even by means of the signs Communion will stand out more clearly as a participation in the sacrifice actually being celebrated.
While the priest is receiving the Sacrament, the Communion chant is begun. Its purpose is to express the communicants' union in spirit by means of the unity of their voices, to show joy of heart, and to highlight more clearly the "communitarian" nature of the procession to receive Communion.
When the distribution of Communion is finished, as circumstances suggest, the priest and faithful spend some time praying privately. If desired, a psalm or other canticle of praise or a hymn may also be sung by the entire congregation.
To bring to completion the prayer of the People of God, and also to conclude the entire Communion Rite, the priest says the Prayer after Communion, in which he prays for the fruits of the mystery just celebrated.
THE CONCLUDING RITES
The concluding rites consist of
a. Brief announcements, if they are necessary;
b. The priest's greeting and blessing, which on certain days and occasions is enriched and expressed in the prayer over the People or another more solemn formula;
c. The dismissal of the people by the deacon or the priest, so that each may go out to do good works, praising and blessing God;
d. The kissing of the altar by the priest and the deacon, followed by a profound bow to the altar by the priest, the deacon, and the other ministers.