The following are several short and sweet answers to various questions concerning the Mass. All citations in the answers are from the new General Instruction of the Roman Missal unless otherwise noted.
Why does the priest kiss the altar?
The altar represents our Lord, who as we pray in the Preface of Easter V “showed Himself to be the priest, the altar, and the lamb of sacrifice." Moreover, traditionally relics of the saints are placed in the altar, which themselves make the altar worthy of such veneration. (Note, “the practice of placing under the altar to be dedicated relics of saints, even of non-martyrs, is to be maintained." #302.)
When can the priest use incense?
The priest may incense the altar and cross after his veneration at the beginning of Mass (#49, #123), the Gospel book before its proclamation (#134), the bread and wine mixed with water after the offertory prayers and the cross (#75, 144); the priest himself because of this office and sacred ministry, and the people, because of their baptismal dignity, may also be incensed by a deacon or other minister (#75). During Easter season, the paschal candle may also be incensed at these times. Since the Old Testament times, incense has been used to purify the sacrifice and the altar itself, and to drive away any presence of evil. Moreover, as stated in the Psalms, the incense signifies our prayers rising up Heaven.
Does a priest have to use a chalice veil?
Traditionally, the chalice and paten are covered with a chalice veil prior to the beginning of Mass. The General Instruction specifies, “It is a praiseworthy practice to cover the chalice with a veil which may be either the color of the day or may be white." (#118). The veil is taken off for the offertory. After the purifications after Communion, the chalice and paten are covered again. Nevertheless, while the practice of using a chalice veil is traditional, it is not mandatory.
Does the priest have to wash his hands during the offertory?
Yes. The priest, after the offertory prayers over the bread and the wine mixed with water, washes his hands, saying, “Lord, wash away my iniquity and cleanse me of my sins" (#76). This gesture is a sign of the priest’s desire for purity of heart as he offers the Sacrifice of the Mass.
Why does the priest mix a little water with the wine in the chalice?
As the priest (or deacon) adds a few drops of water to the wine in the chalice, he prays, “By the mystery of this water and wine, may we come to share in the divinity of Christ, who humbled Himself to share in our humanity." The action thereby symbolizes the hypostatic union of our Lord’s divine and human natures. In the mystery of the incarnation, Jesus Christ, true God, became also true man. Remember too that at the crucifixion, the Roman soldier thrust his lance to the side of Jesus, piercing His Sacred Heart, and from His wounded side flowed Blood and Water (cf. John 19:34). Therefore, the saving benefits of our Lord’s incarnation, passion, death, resurrection, and ascension touch the lives of the faithful and are especially granted through the Holy Eucharist. And it is through the Holy Eucharist, that faithful share in the divine life and love of Jesus who became man for our salvation.
St. Cyprian also suggested that the water represents the Church (the faithful) which is drawn into the life of Christ. As Jesus offered the sacrifice for our sins, shedding His blood, we too are united to Him in His sacrifice as inseparably as water into wine.
Why does the priest drop a particle of the Sacred Host into the chalice during the Lamb of God?
As the priest places a piece of the Sacred Host into the chalice, he prays, “May this mingling of the Body and Blood of our Lord Jesus Christ bring eternal life to us who receive it." This gesture then represents the unity of the Sacrament: the Body, Blood, Soul, and Divinity of Jesus is truly and fully present in both the Precious Blood in the Chalice and the Sacred Host on the paten.
Does a priest have to wear a chasuble?
If the priest is the main celebrant of the Mass, he is required to wear an alb, a stole, and a chasuble. He should also wear a cincture and an amice if needed. (Cf. #337.)
When can the bells be rung?
After the priest pronounces the words of consecration, bells may be rung as he elevates the Sacred Host and then again when he elevates the chalice of Precious Blood. The bells may also be rung “a little before the consecration," usually at the time of the epiclesis. (Cf. #150).