According to General Instruction of the Roman Missal 46, the importance of the Introductory Rites “is to ensure that the faithful who come together as one, establish communion and dispose themselves properly to listen to the Word of God and to celebrate the Eucharist worthily”. 
At St. Mary’s, both at daily Mass and at Sunday Mass, the cantor greets the people and announces the entrance hymn, more properly referred to as the entrance chant. “Its purpose is to open the celebration, foster the unity of those who have been gathered, introduce their thoughts to the mystery of the liturgical time or festivity, and accompany the procession of the Priest and ministers” (General Rites of Instruction 47, cf. 121).  This hymn is carefully selected to fit the themes to be presented in the Mass and involves the active participation of the entire assembly.   The purpose of the cantor at this point is to “direct and support the people’s singing” (GIRM 104).   If a cantor is not present to lead an entrance hymn, the entrance antiphon is recited by the people and should accompany the procession to the altar. 
The Sign of the Cross and Greeting is followed by the Penitential Act.  The penitential act has various forms.  When the Kyrie (Lord Have Mercy) is chanted, the assembly echoes back the chant. A sprinkling rite can replace the penitential act.
The Gloria is sung on Sundays during the season of Ordinary Time and during the Christmas and Easter seasons.  It is omitted during Advent and Lent.  It is also used on feast days or solemnities during the week.  The Gloria may be recited. 
Next week, I will discuss the Liturgy of the Word portion of the Mass in relation to music.