By Communion is meant the actual reception of the Sacrament of the Eucharist. Ascetic writers speak (a) of a purely sacramental reception; that is, when the Eucharist is received by a person capable indeed of the fruits but wanting in some disposition so that the effects are not produced; (b) of a spiritual reception, that is, by a desire accompanied with sentiments of charity; and (c) of a sacramental and spiritual reception, that is, by those who are in a state of grace and have the necessary dispositions. It is of this kind there is question here. For real reception of the Blessed Eucharist it is required that the sacred species be received into the stomach. For this alone is the eating referred to by our Lord (John, vi, 58). Under the moral aspect will be considered, in reference to Holy Communion: necessity; subject; dispositions. The liturgical aspect will embrace: minister of the sacrament; method of administration.
MORAL ASPECT.—-A.--Necessity.—The doctrine of the Church is that Holy Communion is morally necessary for salvation, that is to say, without the graces of this sacrament it would be very difficult to resist grave temptations and avoid grievous sin. Moreover, there is according to theologians a Divine precept by which all are bound to receive Communion at least some times during life. How often this precept urges outside the danger of death it is not easy to say, but many hold that the Church has practically determined the Divine precept by the law of the Fourth Council of Lateran (c. xxi) confirmed by Trent, which obliges the faithful to receive Communion once each year within Paschal Time. B.--Subject.—The subject of Holy Communion is everyone in this life capable of the effects of the Sacrament, that is, all who are baptized and who, if adults, have the requisite intention (see Communion of Children). C.--Dispositions.—That Holy Communion may be received not only validly, but also fruitfully, certain dispositions both of body and of soul are required. For the former, a person must be fasting from the previous midnight from everything in the nature of food or drink. The general exception to this rule is the Viaticum, and, within certain limits, communion of the sick. In addition to the fast it is recommended, with a view to greater worthiness, to observe bodily continence and exterior modesty in dress and appearance. The principal disposition of soul required is freedom from at least mortal sin and ecclesiastical censure. For those in a state of grievous sin confession is necessary. This is the provingoneself referred to by St. Paul (I Cor., xi, 28). The only case in which one in grievous sin might dispense with confession and rest content with perfect contrition, or perfect charity, is where on one hand confession here and now is morally speaking impossible and where, on the other, a real necessity of communicating exists.
LITURGICAL.—A. Minister.—The ordinary minister of Holy Communion is one who has received at least priestly orders. Deacons were often deputed for this office in the early Church. Priests can now by general custom administer Communion to everyone assisting at their Masses in public churches and oratories. For the Viaticum permission of the parish priest is ordinarily required. Communion should be administered to all those who ask it reasonably, excluding, at least until they make sufficient reparation, public sinners and such as lead openly scandalous lives. So, too, it is not to be given to those likely to treat it with irreverence, or to the mentally deranged or those suffering from certain forms of illness.
B.--Method of Administration.—As to the administration, the circumstances of time, place, and manner, and the ceremonies only will be referred to here, other details, as reservation, effects, etc., being considered elsewhere. (See Eucharist.) The ordinary time for administering Communion is during Mass, but any reasonable cause justifies its administration outside Mass, provided it is within the time within which the celebration of Mass is permitted. There are some exceptions: Viaticum can be given at any hour; it is lawful in cases of illness and of special indult. It may not be given except as Viaticum, from the conclusion of the exposition on Holy Thursday till Holy Saturday. Communion may be given in all churches and public, or semi-public, oratories that are not under interdict, and, according to a recent edict of the Congregation of Rites (May 8, 1907), even in domestic oratories to all present. The faithful receive Communion under one kind, fermented bread being used in the Eastern, and unfermented in the Western Church, but priests, who communicate themselves, receive under both kinds. Each one should receive according to the Rite to which he belongs. When administering Holy Communion outside Mass a priest should always wear a surplice and stole, and there should be two lights burning on the altar. Communion may now be given at Masses said in black vestments.
Reference: Catholic Answers