First, a definition: "Holy Communion" is the reception of the Blessed Sacrament (the Eucharist) that has been confected by a priest during the Holy Mass. The Blessed Sacrament may only be received sacramentally by one who:

• is a living human being 
  
  
• is baptized 
   
• has proper intent 
   
• has fasted the proper amount of time: 3 hours is the 1962 practice that most traditional Catholics follow (some follow the older practice of a 12-hour fast); 1 hour is what we are canonically bound to by the 1983 Code of Canon Law. Viaticum -- the "Food for the Journey" given during Extreme Unction -- may be given at any time.  
   
• is in a state of grace, i.e., is not in a state of mortal sin. If one is in a state of mortal sin, he must go to Confession first lest he sin further as St. Paul warns in I Corinthians 11:26-30:

  Therefore whosoever shall eat this bread, or drink the chalice of the Lord unworthily, shall be guilty of the body and of the blood of the Lord. But let a man prove himself: and so let him eat of that bread, and drink of the chalice. For he that eateth and drinketh unworthily, eateth and drinketh judgment to himself, not discerning the body of the Lord. Therefore are there many infirm and weak among you, and many sleep.

  In addition, because Communion is also a sign of Christian unity, those who receive are declaring to the world that they accept all of the dogmas of the Church. Canon 915 of the 1983 Code of Canon Law affirms the Apostolic practice of the Church in insisting that priests refuse Communion to those who are "manifest, obstinate, persistent sinners" -- i.e., those who are public sinners (which includes those who publicly disagree with Church teaching) who refuse to publicly repent -- lest they cause scandal and confuse others as to what Church teaching is. Those who disagree with what the Church teaches should not try to receive Communion.
  
  Summary: one in grave sin is to police himself and refrain from receiving Communion until he's received the Sacrament of Penance. If he fails to, his spiritual father is to advise him, if possible, in order to make him aware of his sin and of the added sin of receiving Communion while not in a state of grace. If, after being advised by his spiritual father, he still fails to police himself, he may be refused the Eucharist if (and only if) the grave sin is a public one (e.g., if he is a heretic or schismatic, if he publicly sins and doesn't publicly repent, if he publicly proclaims positions contrary to the Faith, etc.). Private sins are between the individual, his priest, and God. A priest can't refuse Communion to someone who is guilty of grave sin done privately because a priest can't publicly reveal the private sins of anyone.

The Eucharist must be received at least once a year, during the Easter Season, by those who've reached the age of reason, though frequent -- even daily -- Communion is encouraged. Traditionally, the Eucharist shouldn't be received more than once a day unless it is given as Viaticum during Extreme Unction (the 1983 Code of Canon Law strangely allows for a second reception of the Eucharist "only within a eucharistic celebration in which that person participates.")

The matter of the Sacrament itself are wheat bread made only of flour and water, with nothing added (no honey, no spices, etc; nothing may be added; the use of leavening in the Latin Church is illicit) and wine fermented from grape juice. The former is confected by God through a true priest using the words:

This is My Body

Latin:
Hoc est enim Corpus Meum.

The latter is confected by the words "this is the Chalice of My Blood," which are spoken in the below context in the traditional Mass:

For this is the Chalice of My Blood, of the new and eternal Testament: the Mystery of Faith: which shall be shed for you and for many unto the remission of sins

Latin:
Hic est enim Calix Sanguinis mei, novi et aeterni testamenti: mysterium fidei: qui pro vobis et pro multis effundetur in remissionem peccatorum.

After the substance of bread and wine are changed into His Body and Blood, the accidents -- the appearance, taste, texture of bread and wine -- remain, but what looks like "bread" and "wine" are, in substance, the Body, Blood, Soul, and Divinity of Christ. In other words, outside of Eucharistic miracles which have taken place over 2,000 years, what the eyes and mouth see and taste are the accidents of "bread" and "wine," but what is truly received is Christ and remains Christ until and unless the accidents change such that they are no longer compatible with the species of "bread" and "wine." By this we know, for example, that once the Host goes into one's stomach and is digested, or if a liquid were added to the Precious Blood such that the accidents are no longer recognizable as the accidents of "wine," the Sacrament is no longer there.

Because of the above, it is not okay to refer to the Blessed Sacrament as "bread" or to the Precious Blood as "wine." Once the bread and wine have been consecrated, they are no longer "bread" and "wine"; they are Christ -- Body, Blood, Soul, and Divinity, and the proper words to use to speak of them are "Blessed Sacrament," "Eucharist," "Precious Blood," "Sacred Host" ("Host" comes from the Latin word "hostia" meaning "victim"), etc. The consecrated Hosts and Precious Blood are and will remain Christ regardless of the faith of the people in the pews. They are and will remain Christ whether on the Altar or in the tabernacle or in your mouth or, God forbid, on the floor. They are and will remain Christ for ten years or a thousand years, as long as the accidents of bread and wine remain.

That said, note that the accidents of bread and wine remain even as their substance changes. And the accidents of bread might mold, the accidents of wine might sour, and so forth, even though Christ is truly present. So please note: those who think that receiving Communion incorrectly or if the accidents were to become contaminated brings no risk of transmission of a virus or bacteria -- say, for ex., if two or more people were to drink the Precious Blood from the same chalice -- are wrong.

The Precious Blood is always consumed totally by the priest at the Mass, and the priest will always consume one Sacred Host, distributing others to the people, if present. Remaining Hosts are kept in a ciborium inside the tabernacle between Masses, and this Divine Presence is signalled to us by the sanctuary lamps that burn always outside the tabernacles of our churches and invite us to adore Him and be in His Presence to pray.

The effects of receiving the Sacrament are:

• union, by love, with Christ
   
• an increase in sanctifying grace in the soul when received by a "living member of the Church" (i.e., one who is in a state of grace)
   
• the blotting out venial sin and preserving the soul from mortal sin, in proportion to the communicant's devotion
   
• the rewards promised by Christ in His words, "He that eateth My Flesh and drinketh My Blood, hath everlasting life: and I will raise him up on the last day."

The most proper way to receive the Blessed Eucharist at the altar rail at Mass is to kneel, keep your eyes downcast, and fold your hands in the "prayer" gesture at about mid-chest level, no higher (or place them under the houseling cloth at the altar rail, if such a cloth is used; don't touch the cloth or the rail in either case). When the priest reaches you and it is time for you to receive Christ, an acolyte or altar boy will hold a paten underneath your chin so that no precious particles will fall to the floor. The priest will bless you by making a Sign of the Cross with the Sacrament (a small one in the air) and then place the Sacrament on your tongue, all while saying these words:

Tilt your head backwards slightly (say twenty degrees), open your mouth, stick out your tongue far enough so the priest can place the Host on your tongue, and don't move at all. Many priests recommend closing your eyes when receiving so that you're not tempted to follow your priest's hand. And, of course, don't close your mouth until the Host is safely on your tongue and the priest's hand is out of the way. Don't say "Amen" or anything else after receiving. Just make the Sign of the Cross, then return to your pew and kneel in thanksgiving (many people cover their faces with their hands or veils at this time to increase a sense of intimacy).

Please note that the Eucharist is not chewed, but is allowed to soften in the mouth and then swallowed. ¹ This is to avoid having the smallest particle stuck in one's teeth where it might be desecrated later by coming into contact with the profane. Having the very Body, Blood, Soul, and Divinity of Christ mingle with a gulp of Mountain Dew is hardly the treatment He deserves -- but pondering the very possibility of such a thing is to induce gratitude for the amazing humility with which He comes to us under the appearance of bread; why, if He were to come to us in a way that revealed His glory to our senses, we would no doubt die from being in the Presence of such obvious Holiness.

If, God forbid, the Host is dropped, don't move or say anything. The priest and acolytes will move into action, retrieving every particle. Everything will stop until this work is finished. Just stay in place. To help prevent one source of accidents like this, if you're carrying a baby when you receive, hold the child's hands when the priest is giving you Communion.

The Eucharist should never be touched but by consecrated hands (i.e., the hands of a priest, who is the ordinary minister of the Sacrament, or of a delegated deacon, who is the extraordinary minister of the Sacrament) unless emergency or true charity dictate otherwise, and women should have their heads covered whenever they are in His Presence -- whether during simple visits to a church where the tabernacle is, during sick calls or Unction, during Eucharistic processions, and when receiving the Eucharist at Mass.

In the Novus Ordo, many non-Catholics and Catholics who aren't receiving Communion will go up to the priest with arms crossed in order to receive a priestly blessing. This isn't done during the traditional Mass. If you're not receiving, just remain in your pew.

A beautiful, traditional, partially indulgenced prayer to pray after receiving Communion is the "Anima Christi" (Soul of Christ) -- a favorite prayer of St. Ignatius of Loyola. The origins of this ancient prayer are unknown, but it dates to at least A.D. 1334.
 

First Communion

 
If one enters the Church as an adult, First Communion is usually given on the same day of Baptism and Confirmation (which both take place, typically but not necessarily, during the Easter Vigil).

If one grows up in the Church, First Communion is offered at the discretion of one's priest. It may be given to a lone child after the priest has discerned that the child understands the Sacrament and is able to form proper intent, or it may be given to a group of children who've been properly prepared together, such as a first grade class. In either case, the Sacrament of Confession is received first before Communion.

Little boys will dress in their finest suits and each wear a white rosette pinned to their lapels, and little girls will often wear special white First Communion dresses and veils (their dresses should fit the rules of feminine modesty in Church -- nothing sleeveless, etc.). On a mundane, sociological level, a child's "First Communion" is a rite of passage, an acknowledgement that he has reached the age of reason and is now liable for many of the penalties involved in ecclesiastical censure; it is, in other words, a marker that the child is growing up...

Gifts are given to the new communicant (typically Rosaries, prayer books, Bibles, etc.), and a party typically follows the Mass during which he first receives the Sacrament. These mundane aspects of a child's "First Communion" should never overshadow the greater reality! In some parts of the world, a child's First Communion is turning into a lavish, extravagant, keeping-up-with-the-Joneses bat-mitzvah, with little girls, wearing dresses that cost hundreds and hundreds of dollars, carried about by limousine to parties with expensive ice sculptures and la-di-da waiters. It is disgusting!

While a child's First Communion should be memorable and very beautiful, it should, above all, be holy and with all priorities in place. On the more fundamental and profound level, First Communion is an initiation into one of the Great Mysteries. Parents should prepare the child by firmly grounding him in basic catechesis. While it's the priest's decision as to whether or not your child is adequately prepared, it is your job as a parent or godparent to do the preparing; it is the parents and godparents who are ultimately responsible for the Catholic education of the child. The child should understand what transubstantiation is. He should know that God, Who created all things -- the Sun and Moon and Stars -- is able to speak things into reality, and that at the Mass, this is what God, through His priest does. The child should understand that though the accidents of bread and wine remain, what the bread and wine truly become is Sacrament. He will learn all of this best by watching the adults around him, especially parents and godparents, who should ask themselves:  

• Do I treat and speak of the Blessed Sacrament with reverence?
• Do I kneel toward the tabernacle when I enter a church?
• Do I bow my head and cross myself when passing by a church to honor the Real Presence of Christ in the tabernacle?
• Do I fast before receiving Communion?
• Am I sure to never receive the Sacrament in a state of mortal sin?
• Do I make visits to the Blessed Sacrament?
• Do I allow the children to attend a parish in which lay people sacrilegiously handle the Sacrament as "extraordinary Eucharistic ministers"?
• Do I allow the children to attend Masses wherein the rubrics and prayers after consecration destroy faith in the Real Presence?
• Do I attend Protestant worship services? If I do, for the cause of charity, do I go through the motions of receiving "communion" at the services of Protestant faith communities? (Catholics may not attend Protestant services except for attending weddings in which neither of the couple had ever been Catholic, and attending funerals, which Catholics may certainly attend out of charity. Never may a Catholic eat the bread/crackers or drink the wine/juice offered during the services, and in no case may he join in prayers that are in no way Catholic.)

Your children will learn more from your example than anything else.

As to preparation for the Rite itself, parents and godparents should consider the natural intimidation that most children experience in such formal circumstances (especially if the child is receiving his First Communion alone) and affirm their child emotionally, letting them know it's OK to be nervous. A "practice-run" with everyday, ordinary bread might be helpful, ² with the parent or godparent showing the child the proper posture and gesture. Anticipate the child's questions ("What will it taste like?" for example) and encourage the child to express any concerns and fears he might have. Teach him to pray the sentiments expressed in the Anima Christi prayer, if not the prayer itself, after receiving the Host. Perhaps getting a holy card that contains this prayer, or writing the prayer out for him on a small piece of paper so he can refer to it after Communion will help.

I'll note here, too, that Pope St. Pius X, the "Pope of the Eucharist," is one of the patrons of First Communicants as it was he who especially encouraged frequent and early Communion -- as soon as a child is able to understand the Sacrament -- in the Latin Church. Teach your child about this great Pope and encourage him to pray to him, asking St. Pius X to intercede in making your child's First Communion most fruitful. One standard prayer to this holy man is this one, the first part of which is most appropriate to the day:

Glorious Pope of the Eucharist, St. Pius X, who sought to restore all things in Christ, obtain for me a true love of Jesus that I may live only for Him. Help me, that, with lively fervour and a sincere will to strive for sanctity of life, I may daily avail myself of the riches of the Holy Eucharist in Sacrifice and Sacrament. By your love for Mary Mother and Queen of all, inflame my heart with tender devotion to her.

Blessed model of the priesthood, obtain for us holy and zealous priests and increase vocations to the religious life. Dispel heresy and incline hearts to peace and concord, that all nations may place themselves under the sweet reign of Christ. Amen.

St. Pius X, pray for me.

Another patron of First Communicants is Blessed Imelda Lambertini (A.D. 1322-1333), who died while receiving her First Holy Communion. Very much in love with Jesus, she'd begged her family to let her live at the Dominican convent at the age of nine. Her family relented, as did the Dominicans, but she still could not yet receive Communion. She longed for it, however, and watching the Sisters receiving Our Lord, would pray for spiritual Communion. One day -- it was the Vigil of the Ascension -- she was making her spiritual Communion, and the Sisters saw a beautiful light glowing over her, and a Host at the center of it, hovering above her head. The priest was summoned, and Imelda received the Eucharist at once -- but in such an ecstasy that she literally died of love. Her relics can be venerated in the Church of Saint Sigismondo in Bologna. Italy (see relics page for a picture). Pope St. Pius X made her a Patroness of First Communicants. Here, in pdf format, is the story of Blessed Imelda for your children. Use your discretion, as the story might frighten young ones who don't understand death and ecstasies.

 
 
 
Footnotes:
1 If you take medications that dry the mouth, or otherwise lack enough saliva to swallow the Host without chewing, go ahead and chew. We're not Pharisees.

² A little American Catholic cultural trivia: some Catholic schools used to teach their First Communion classes using NECCO ® Wafers candies made by the New England Confectionery Co. (hence the name "NECCO") as "hosts" for practice. Catholic children often use them to "play Mass," too, and many a priest has memories of using the candies that way as a child. The chalky-sweet wafers come in 8 flavors, all mixed in a single roll: lemon (yellow); orange (orange); lime (green); clove (purple); cinnamon (white); wintergreen (pink); licorice (black); and chocolate (brown). The New England Confectionery Company, by the way, is the same company that makes "Mary Janes" and "Conversation Hearts" -- those little hearts, created in 1866 and imprinted with love messages such as "Be Mine" or "Kiss Me," that are sold for St. Valentine's Day.