The norms for Communion services are provided in Eucharistiae Sacramentum
([ES]—On Holy Communion and Worship of the Eucharist Outside Mass), a decree which the Congregation for Divine Worship (CDW) issued in 1973.
Given that priests and deacons are ordinary ministers of the Holy Eucharist, and when no priest is available to offer Mass, a deacon is permitted to conduct a Communion service, and should be the person who leads it if he is available and has been authorized to do so.
In the absence of a deacon, a Communion service may be led by an acolyte, who is a member of the lay faithful:
It is the office of an acolyte who has been properly instituted to give Communion as a special minister when the priest and deacon are absent or impeded by sickness, old age, or pastoral ministry, or when the number of the faithful as the holy table is so great that the Mass or other service may be unreasonably protracted (ES 17).
To be clear, an acolyte does not refer to an extraordinary minister of Holy Communion (EMHC) or an altar boy. It refers to a specific, liturgical office historically held by men, and usually in preparation for the priesthood. In 2021, Pope Francis authorized women to serve as acolytes).
In Eucharistiae Sacramentum, the CDW also granted diocesan bishops and those whom they depute the power to authorize other lay people to serve at Communion services:
The local Ordinary may give other special ministers the faculty to give Communion whenever it seems necessary for the pastoral benefit of the faithful and a priest, deacon, or acolyte is not available (ES 17).
A Communion service follows a structure similar to the Mass. Because a priest is not present, the prayers reserved to him are excluded (e.g., the Canon of the Mass). Other prayers are modified to reflect the role of the one presiding (e.g., the Penitential Rite and Final Blessing).
After the Penitential Rite, “the Liturgy of the Word now takes place as at Mass” (ES 29).
Because Mass is not celebrated, there is no sacramental re-presentation of Christ’s one Sacrifice of Calvary. Consecrated Hosts are taken from the tabernacle right before the assembly recites the Lord’s Prayer (Our Father).
After the sign of peace, the minister takes the Host, “raises it slightly over the vessel or pyx and, facing the people, says, ‘This is the Lamb of God who takes away the sins of the world. Happy are those who are called to his supper'” (ES 32).
How frequently may weekday Communion services be conducted?
Because of abuses that took place in the celebration of the Communion services, the Congregation for Divine Worship and the Discipline of the Sacraments (CDWDS) issued new norms in its 2004 instruction Redemptionis Sacramentum ([RS] On Certain Matters to Be Observed or to Be Avoided Regarding the Most Holy Eucharist).
The Church sees Communion services during the week when there is no priest as an extraordinary exception rather than the norm:
Likewise, especially if Holy Communion is distributed during such celebrations, the diocesan bishop, to whose exclusive competence this matter pertains, must not easily grant permission for such celebrations to be held on weekdays, especially in places where it was possible or would be possible to have the celebration of Mass on the preceding or the following Sunday (RS 166).
Even on Sundays, the Code of Canon Law recommends a simple Liturgy of the Word rather than a Communion service:
If participation in the eucharistic celebration becomes impossible because of the absence of a sacred minister or for another grave cause, it is strongly recommended that the faithful take part in a liturgy of the word if such a liturgy is celebrated in a parish church or other sacred place according to the prescripts of the diocesan bishop or that they devote themselves to prayer for a suitable time alone, as a family, or, as the occasion permits, in groups of families (canon 1248 §2).
In addition, in the Directory for Sunday Celebrations in the Absence of a Priest, which the Congregation for Divine Worship issued in 1988, the Church provides:
It belongs to the diocesan bishop, after hearing the [diocesan] council of presbyters, to decide whether Sunday assemblies without the celebration of the Eucharist should be held on a regular basis in his diocese. It belongs also to the bishop, after considering the place and persons involved, to set out both general and particular norms for such celebrations. These assemblies are therefore to be conducted only in virtue of their convocation by the bishop and only under the pastoral ministry of the pastor (24).
In Redemptionis Sacramentum, the CDWDS further adds:
It is necessary to avoid any sort of confusion between this type of gathering and the celebration of the Eucharist. The diocesan Bishops, therefore, should prudently discern whether Holy Communion ought to be distributed in these gatherings (RS 165).
In summary, although the Church does foresee a Liturgy of the Word with Communion (i.e., a Communion service) as a possibility on a Sunday, the Church directs that such liturgies be rarely permitted on weekdays.
No presiders at Communion service; multiple people should have parts; those who preach must be authorized to do so
In addition, there are other caveats regarding the conducting of Communion services:
It will be preferable, moreover, when both a priest and a deacon are absent, that the various parts be distributed among several faithful rather than having a single lay member of the faithful direct the whole celebration alone. Nor is it ever appropriate to refer to any member of the lay faithful as “presiding” over the celebration (RS 165).
Finally, those who preach at Communion services must be authorized by the bishop or one he deputes:
The homily on account of its importance and its nature is reserved to the priest or deacon during Mass. As regards other forms of preaching, if necessity demands it in particular circumstances, or if usefulness suggests it in special cases, lay members of Christ’s faithful may be allowed to preach in a church or in an oratory outside Mass in accordance with the norm of law. . . . All must remember besides that the faculty for giving such permission belongs to the local ordinary, and this as regards individual instances; this permission is not the competence of anyone else, even if they are priests or deacons (RS 161, emphasis added).