After the Prayer of the Faithful, while the priest remains at the chair, the deacon prepares the altar, assisted by the acolyte, but it is the deacon’s place to take care of the sacred vessels himself. He also assists the priest in receiving the people’s gifts. Next, he hands the priest the paten with the bread to be consecrated, pours wine and a little water into the chalice, saying quietly, Per huius aquae (By the mystery of this water), and after this presents the chalice to the priest. He may also carry out the preparation of the chalice at the credence table. If incense is used, the deacon assists the priest during the incensation of the gifts, the cross, and the altar; afterwards, the deacon himself or the acolyte incenses the priest and the people.
During the Eucharistic Prayer, the deacon stands near the priest but slightly behind him, so that when needed he may assist the priest with the chalice or the Missal. From the epiclesis until the priest shows the chalice, the deacon normally remains kneeling. If several deacons are present, one of them may place incense in the thurible for the consecration and incense the host and the chalice as they are shown to the people.
At the final doxology of the Eucharistic Prayer, the deacon stands next to the priest, holding the chalice elevated while the priest elevates the paten with the host, until the people have responded with the acclamation, Amen.
After the priest has said the prayer at the Rite of Peace and the greeting Pax Domini sit semper vobiscum (The peace of the Lord be with you always) and the people have responded, Et cum spiritu tuo (And also with you), the deacon, if it is appropriate, invites all to exchange the sign of peace. He faces the people and, with hands joined, says, Offerte vobis pacem (Let us offer each other the sign of peace). Then he himself receives the sign of peace from the priest and may offer it to those other ministers who are closer to him.
After the priest’s Communion, the deacon receives Communion under both kinds from the priest himself and then assists the priest in distributing Communion to the people. If Communion is given under both kinds, the deacon himself administers the chalice to the communicants; and, when the distribution is completed, he immediately and reverently consumes at the altar all of the Blood of Christ that remains, assisted if necessary by other deacons and priests.
When the distribution of Communion is completed, the deacon returns to the altar with the priest and collects the fragments, if any remain, and then carries the chalice and other sacred vessels to the credence table, where he purifies them and arranges them in the usual way while the priest returns to the chair. It is also permissible to leave the vessels that need to be purified, suitably covered, at the credence table on a corporal, and to purify them immediately after Mass following the dismissal of the people.