The Code of Canon Law (#917) stipulates, “A person who has received the Most Holy Eucharist may receive it again on the same day only during the celebration of the Eucharist in which the person participates, with due regard for the prescription of Canon 921, §2." Following this lead, Canon 921, §2 stipulates, “Even if they have received Communion in the same day, those who are in danger of death are strongly urged to receive again." Succinctly, a person may receive Holy Communion twice a day.
Given this citing of official Church law, we must appreciate the rationale that serves as its foundation. The Holy Sacrifice of the Mass and the celebration of the Blessed Sacrament is “the true center of the whole Christian life both for the universal Church and for the local congregation of that Church" (Instruction on the Worship of the Eucharistic Mystery, #6). The offering of the Mass and the reception of Holy Communion are intrinsically connected. Moreover, the components of the Mass, particularly the Liturgy of the Word and the Liturgy of the Eucharist, form a cohesive whole.
Therefore, in ordinary circumstances, an individual is obligated to participate fully in the entire Mass making an offering of oneself to the Lord. The person ought to be present from the very beginning until the very end of the Mass, giving full attention as best one can. Such full participation and attention dispose the individual to receive Holy Communion. Worthy reception of Holy Communion not only enables the individual to enter into communion with the Lord, but also binds that individual in a communion of faith and love with the other members of the Church.
Never, in ordinary circumstances, do we isolate the reception of Holy Communion from the rest of the Mass. The Church grants the permission of receiving Holy Communion twice in one day to meet those situations of a person attending perhaps a wedding Mass and a funeral Mass on the same day, or attending the regular daily Mass and then some special Mass the same day; nevertheless, the stipulation is that the person attends the whole Mass in both instances. Unfortunately, I have known individuals who on a daily basis just “pop" into Mass (even Masses) at the right time to receive Holy Communion and then leave before Mass concludes; it is almost like they are getting their “Jesus fix" for the day rather than worshiping God and wholeheartedly receiving the Blessed Sacrament.
As qualified in Canon 921, §2, in those special circumstances when a person is in danger of death, then he may receive Holy Communion as viaticum along with Penance and Anointing of the Sick, even though he may have received twice already that day. Another special circumstance arises when the person is confined in a hospital or homebound: here the person may receive outside the context of Mass, but would not receive more than once a day unless in danger of death.
Two other basic stipulations govern the reception of Holy Communion: First, a person who is conscious of mortal sin must first make a sacramental confession and receive absolution. If no legitimate opportunity exists for first going to confession, then a person may make an act of perfect contrition with the pledge to the Lord to go to Penance as soon as possible before receiving Holy Communion (Code of Canon Law, #916).
Second, a person must fast from food and drink (except water or medicine) for one hour beforehand (Code of Canon Law, #919). However, the period of fast before receiving Holy Communion is reduced to “approximately one quarter of an hour" for those who are sick at home or at a hospital, those elderly confined to home or a nursing home, and those who care for these people and who are unable conveniently to observe the fast (Immensae Caritatis, 1973).
The Church in her prudence provides these laws to help us have a balanced spiritual life, avoiding extremes. Just as the Church requires a person to receive Holy Communion at least once a year (the “Easter duty law"), so does the Church restrict the number of times we can receive a day.