Welcome to this short introduction to the celebration of the Holy Mass, or Eucharist (which means thanksgiving)! The Eucharist is the source and summit of the Christian life and its highest and most important prayer. In the Mass the whole mystery of our salvation is made present, and in the Eucharist we receive the Body and Blood of Jesus our Lord, who is really and truly present under the appearance of bread and wine and who gives himself to us in this great gift of love.
This short guide will help you to understand what the Mass is, so that you can better prepare yourself to take a part in its celebration, by answering the following questions: What is the origin of the Mass? What exactly is the Eucharist? What are the fruits of Holy Communion in the lives of those who receive it? What are the main parts of the Mass? Who can and who cannot receive Holy Communion?
The signs of bread and wine, which signify the goodness of creation, go all the way back to Melchizedek, priest of God most high, who brought an offering of bread and wine to Abraham (Gen 14:18). In the Old Testament bread and wine were offered in sacrifice among the first fruits of the earth as a sign of grateful acknowledgement to the Creator. 
At the time of the Exodus, the deliverance out of Egypt began when the Israelites slaughtered a young lamb, put its blood on the doorpost of their houses, and then had to eat the Lamb for them to be protected from the angel of death (Ex 12). The Exodus also gave a special significance to the signs of bread and wine: the unleavened bread that Israel eats every year at Passover commemorates the haste of their departure from Egypt; the remembrance of the manna in the desert recalls to Israel that it lives by the bread of the Word of God. And the “cup of blessing” at the end of the Passover meal adds to the festive joy of wine the messianic expectation of the rebuilding of Jerusalem. 
When Jesus began his public ministry, John the Baptist called him the “Lamb of God” (John 1:36) – a name that pointed to his future self-offering as a sacrifice for our sins. Jesus also called himself the “living bread that came down from heaven” and promised that they who would eat this bread would live forever (John 6:51). He said: “Unless you eat the flesh of the Son of Man and drink his blood, you have no life in you. Whoever eats my flesh and drinks my blood has eternal life, and I will raise him up at the last day. For my flesh is food indeed, and my blood is drink indeed” (John 6:53-55). This was such a shocking teaching that many of his disciples were scandalized and left him after hearing it (Jn 6:66) – and so the Eucharist and the cross remain stumbling blocks and an occasion of division even today. Yet the Lord asks us: “Will you also go away?” (John 6:67) as a loving invitation to discover that only he has the words of eternal life and that to receive in faith the gift of the Eucharist is to receive the Lord himself and the eternal life he has come to share with us.
On the eve of his passion and death, Jesus ate his last Passover meal with his apostles, and he transformed this Last Supper into the memorial of his voluntary offering to the Father for the salvation of the world. By instituting the Eucharist he also instituted a New Covenant with the house of Israel, giving a new and definitive meaning to the blessing of the bread and the cup by saying: “This is my body… this is my blood of the new covenant, which is shed for many for the remission of sins” (Mt 26:26-27). The following day, Jesus’ death on the cross became the definitive Paschal sacrifice for the redemption of mankind and the sacrifice of the New Covenant which reconciles man to God and restores man to communion with Him.
At the Last Supper, Jesus told his apostles to “do this in remembrance of me” (Luke 22:19) – that is to repeat his actions and words until he returns. This does not only mean to remember what Jesus did; it also points to the liturgical celebration, by the apostles and their successors, of Christ’s life, death and resurrection.
When we celebrate the memorial of the Lord’s sacrifice, we offer to the Father what he has himself given us: the gifts of his creation, bread and wine which have become the body and blood of Christ. The Eucharist is therefore:
The Eucharist is God’s greatest gift to us. Throughout the history of salvation, God has always wished to be present with us, to dwell with us and even in us – to be “Immanuel” – God is with us. In the Old Testament, his Real Presence dwelt in the Ark of the Covenant in the Holy of Holies, but nobody could approach it because of the gap between God’s holiness and the people’s sinfulness. When Jesus came to reconcile us with the Father, he opened the way for us into the Holy of Holies: now not only we can approach God freely as our Father, we can also receive into us Him who has loved us to the end. And so the Eucharist is also a joyful banquet of communion and intimate union of the faithful with Christ himself who has offered himself to us.
The Eucharist is usually kept in the tabernacle of most Catholic churches – a small box at the front of the church usually with a lit candle besides it to indicate the Lord’s presence. The name 'tabernacle' reminds us of the tabernacle in the desert, God's dwelling among His people Israel as they wandered through the desert towards the Promised Land. In those times God dwellt in the Holy of Holies and was inapproachable except for once a year by the high priest. Now His Real Presence is accessible to all in the tabernacle of every Catholic Church. When we stand before His Presence in the tabernacle, it is as if we stand before the Holy of Holies of the ancient tabernacle. Because God is truly present in the Eucharist, we usually make a sign of reverence before the tabernacle, either by genuflecting or bowing. Jesus also invites us and awaits us in this sacrament of love to come pray and adore him. Quiet time before the Lord’s presence in the Eucharist is one of the best ways to deepen our relationship with him and to find quiet and peace from the troubles and worries of everyday life.
What does the Eucharist do to those who receive it frequently?
Since the earliest days of the Church, the Mass has kept the same basic structure. It is composed of two principal parts:
In the Liturgy of the Word, we listen to the Word of God from the Bible and respond to it with acclamations and a psalm. Hearing God’s Word increases our faith and understanding of the mystery of salvation before we take a direct part in it through the Eucharist.
In the Liturgy of the Eucharist, the elements of bread and wine are brought to the altar, and through the Eucharistic prayer of the priest(s) they become the body and blood of the Lord Jesus who wishes to give himself to us. 
[Parts in brackets are said on Sundays and feast days only]
Only validly ordained priests can preside at the Eucharist and consecrate the bread and the wine so that they become the Body and Blood of the Lord.  When Jesus instituted the Eucharist, not all of his disciples were present but only his twelve apostles. Although all believers in Jesus form a “kingdom of priests,” Jesus only gave to the apostles and to their successors (bishops and priests) the authority to celebrate the Eucharist in his name – just as in the Old Testament only the Aaronic priests could offer sacrifices even though all of Israel was a “kingdom of priests and a holy nation” (Ex 19:6).
We welcome Catholics to participate fully in the celebration of the Eucharist by receiving Holy Communion in fulfillment of Christ's command to eat His body and drink His blood (Jn 6:51). In order to be properly disposed to receive Communion, communicants should not be conscious of grave sin, have fasted for an hour before Communion, and seek to live in charity and love with their neighbors. Those conscious of grave sin must first be reconciled with God and the Church through the sacrament of Penance. A frequent reception of the sacrament of Penance is encouraged for all. 
Attending Mass on Saturday evening or Sunday (and on important feast days) is obligatory for Catholics, in fulfillment of the third (or fourth) commandment to sanctify the Sabbath (or Lord’s Day). A Catholic who deliberately misses Mass on these days commits a grave sin and should receive the sacrament of Penance before he returns to the table of the Eucharist. 
We welcome you to attend the celebration of the Eucharist as our brothers and sisters in Messiah, even though we do not yet have full unity and communion between us. We pray that our common baptism and the action of the Holy Spirit in this Eucharist will draw us closer to one another and begin to dispel the sad divisions which separate us. We pray that these will lessen and finally disappear, in keeping with Christ’s prayer for us “that they may all be one” (Jn 17:21).
Because the Eucharist is a sign of the oneness and unity of our faith, life and worship, we unfortunately cannot extend to you an invitation to receive Communion.  Since non-Catholic believers do not usually share our faith regarding the nature of the Church, the priesthood, and the Real Presence of Christ in the Eucharist, sharing Communion together would imply a oneness which does not yet exist, and for which we must all pray. If you would like to learn more about the Catholic faith and how you could receive the Eucharist, please ask a priest or a helper and they will gladly answer your questions.
We also welcome to this celebration those who do not share our faith in Jesus Christ. While we cannot admit them to Holy Communion, we ask them to offer their prayers for the peace, unity, conversion and salvation of the human family.
All who are not receiving Holy Communion are encouraged to express in their hearts a prayerful desire for unity with the Lord Jesus and with one another.