The short answer to this question is twice in one day. A fuller explanation, though, will uncover not only how often we may receive, but more simply how we should approach the Most Blessed Sacrament.
St. Paul instructs the Corinthians that before receiving Holy Communion, they should discern if they are worthy to receive (1 Corinthians 11:27-29). The Church has interpreted this to mean an examination of our conscience, ensuring that we have not committed any mortal (grave) sin since our last confession. If we have, we should first go to confession before going to Communion.
Also, whenever we receive Communion, we are instructed by the Church to fast from food and drink, other than water, for one hour prior to receiving Communion. This exterior act prepares us to receive. Our desire for something to eat or drink during this period reminds us of our deeper desire to receive God in the Eucharist.
This desire to receive God in his heart moved one monk to celebrate Holy Mass at noon each day. From the time he woke up, he prayed, longing to receive Communion. At noon, he celebrated Mass, receiving the Lord, whom his heart sought. The rest of the day, he rejoiced that he was able to receive Communion that day. The purposeful timing of Mass made it the high point of his day, the point his day pointed toward and from which it unfolded.
How this monk treated his reception of Communion is how we are called to receive as well. It is the high point of our day, no matter how early or late in the day we may receive.
Thus, receiving Communion twice in one day is not about receiving more of Jesus or walking in at Communion just to receive. It is meant to help us more fully participate in the celebrations that mark the high points of our lives. Along these lines, one of the most common instances one may receive Communion twice in one day is when one goes to a wedding, funeral, another special Mass, or a daily Mass at one time and then attends a special Mass (e.g. wedding or a funeral) or the Saturday evening vigil. Another instance may be if we must minister in one role at one Mass and then in another at another Mass later that day.
A final common instance would be if one went to Communion at Mass and then became gravely ill and received again. In these and similar cases, it would be permissible to receive Communion twice in one day, bearing in mind that whenever we approach the Eucharist, we come with great preparation and intention. In so doing, we manifest respect to Jesus whom we are receiving, and receive all that He has to give to us.
Father Mayo is pastor at St. Raphael Parish in south St. Louis.