Scripture and the Early Christian Writings give us an idea of the communal worship of the earliest Christians... they "broke bread", read Scriptures, gave money for the benefit of the congregation, received a blessing.
OT backdrop: In Genesis 14 Abram meets with the Priest of God, Melchizedek. The priest shares bread & wine with him and gives God's blessing and Abram gives a tithe of his income.
17 And the king of Sodom went out to meet him after his return from the slaughter of Chedorlaomer, and of the kings that were with him, at the valley of Shaveh, which is the king’s dale.
18 And Melchizedek king of Salem brought forth bread and wine: and he was the priest of the most high God.
19 And he blessed him, and said, Blessed be Abram of the most high God, possessor of heaven and earth:
20 And blessed be the most high God, which hath delivered thine enemies into thy hand. And he gave him tithes of all.
21 And the king of Sodom said unto Abram, Give me the persons, and take the goods to thyself.
22 And Abram said to the king of Sodom, I have lift up mine hand unto the Lord, the most high God, the possessor of heaven and earth,
Malachi says that in the future (relative to that prophet's time) a perfect sacrifice will be offered repeatedly, throughout the world, among all nations, until the end of time...
Malachi 1
11 For from the rising of the sun even unto the going down of the same my name shall be great among the Gentiles; and in every place incense shall be offered unto my name, and a pure offering: for my name shall be great among the heathen, saith the Lord of hosts.
That prophecy is fulfilled in the institution and continuing daily celebration of the Lord's Supper. In every region of the world, every day, a Mass is celebrated, which Catholics believe is the actualization, the re-presenting, of the once-in-time sacrifice of the Lamb of God: Jesus.
The Gospels of Matthew,Mark and Luke (sometimes referred to as the "synoptic" gospels) describe Jesus celebrating/instituting the New Passover feast, He Himself being the perfect Lamb of God which is sacrificed for us.
Acts 2
42 And they continued stedfastly in the apostles’ doctrine and fellowship, and in breaking of bread, and in prayers.
Here we see that in the common worship they studied the Apostolic teachings together, prayed together and "broke bread" together. To "break bread" means to celebrate the Lord's Supper. To just eat for nourishment they used a phrase like "eat meat" or "take food", as in Acts2:46...
46 And they, continuing daily with one accord in the temple, and breaking bread from house to house, did eat their meat with gladness and singleness of heart,
Acts 20
7 And upon the first day of the week, when the disciples came together to break bread, Paul preached unto them, ready to depart on the morrow; and continued his speech until midnight.
So, in the Apostolic age, the Christians gathered together on Sunday (the first day of the week) to read the Apostles' writings and the OT Scriptures and hear preaching and worship God and celebrate the memorial of the Last Supper.
According to early Christian writings of the late- and post-Apostolic era, the Last Supper memorial was the re-presentation of the sacrifice of the Lamb of God. The bread and wine, they believed, actually became the flesh & blood of that Passover Lamb and they ate that Passover Lamb (Jesus), as directed by God in Exodus and by Jesus in the Gospels. (The original Passover Lamb meal foreshadowed the New Covenant Passover meal which is the Eucharist (the Lord's Supper) as instituted by Christ).
The following text extracts, from the first & second centuries, describe the weekly Sunday worship. Without the attributions of the sources, you might think these came from the modern catechism or from someone's description of the modern Catholic Mass...
Didache (circa 100 AD) Ch 14
But every Lord's day gather yourselves together, and break bread, and give thanksgiving after having confessed your transgressions, that your sacrifice may be pure. But let no one that is at variance with his fellow come together with you, until they be reconciled, that your sacrifice may not be profaned. For this is that which was spoken by the Lord: In every place and time offer to me a pure sacrifice; for I am a great King, says the Lord, and my name is wonderful among the nations.
Justin Martyr (100 - 165 AD)
First Apology Ch 66
And this food is called among us Εὐχαριστία [the Eucharist], of which no one is allowed to partake but the man who believes that the things which we teach are true, and who has been washed with the washing that is for the remission of sins[Baptism], and unto regeneration, and who is so living as Christ has enjoined. For not as common bread and common drink do we receive these; but in like manner as Jesus Christ our Saviour, having been made flesh by the Word of God, had both flesh and blood for our salvation, so likewise have we been taught that the food which is blessed by the prayer of His word, and from which our blood and flesh by transmutation are nourished, is the flesh and blood of that Jesus who was made flesh. For the apostles, in the memoirs composed by them, which are called Gospels, have thus delivered unto us what was enjoined upon them; that Jesus took bread, and when He had given thanks, said, This do in remembrance of Me, Luke 22:19 this is My body;
and that, after the same manner, having taken the cup and given thanks, He said, This is My blood;
and gave it to them alone.
Ch 67
And we afterwards continually remind each other of these things. And the wealthy among us help the needy; and we always keep together; and for all things wherewith we are supplied, we bless the Maker of all through His Son Jesus Christ, and through the Holy Ghost. And on the day called Sunday, all who live in cities or in the country gather together to one place, and the memoirs of the apostles or the writings of the prophets are read, as long as time permits; then, when the reader has ceased, the president verbally instructs, and exhorts to the imitation of these good things. Then we all risetogether and pray, and, as we before said, when our prayer is ended, bread and wine and water are brought, and the president in like manner offers prayers and thanksgivings, according to his ability, and the people assent, saying Amen; and there is a distribution to each, and a participation of that over which thanks have been given, and to those who are absent a portion is sent by the deacons. And they who are well to do, and willing, give what each thinks fit; and what is collected is deposited with the president, who succours the orphans and widows and those who, through sickness or any other cause, are in want, and those who are in bonds and the strangers sojourning among us, and in a word takes care of all who are in need. But Sunday is the day on which we all hold our common assembly, because it is the first day on which God, having wrought a change in the darkness and matter, made the world; and Jesus Christ our Saviour on the same day rose from the dead. For He was crucified on the day before that of Saturn (Saturday); and on the day after that of Saturn, which is the day of the Sun, having appeared to His apostles and disciples, He taught them these things, which we have submitted to you also for your consideration.
Dialogue with Trypho, Ch 41
[Trypho was a Jewish man Justin was teaching or lecturing or debating or something...]
Hence God speaks by the mouth of Malachi, one of the twelve [prophets], as I said before, about the sacrifices at that time presented by you: 'I have no pleasure in you, says the Lord; and I will not accept your sacrifices at your hands: for, from the rising of the sun unto the going down of the same, My name has been glorified among the Gentiles, and in every place incense is offered to My name, and a pure offering: for My name is great among the Gentiles, says the Lord: but you profane it.' Malachi 1:10-12 [So] He then speaks of those Gentiles, namely us, who in every place offer sacrifices to Him, i.e., the bread of the Eucharist, and also the cup of the Eucharist, affirming both that we glorify His name, and that you profane [it].
Dialogue with Trypho, Ch 117
Accordingly, God, anticipating all the sacrifices which we offer through this name, and which Jesus the Christ enjoined us to offer, i.e., in the Eucharist of the bread and the cup, and which are presented by Christians in all places throughout the world, bears witness that they are well-pleasing to Him....
For there is not one single race of men, whether barbarians, or Greeks, or whatever they may be called, nomads, or vagrants, or herdsmen living in tents, among whom prayers and giving of thanks are not offered through the name of the crucified Jesus. And then, as the Scriptures show, at the time when Malachi wrote this, your dispersion over all the earth, which now exists, had not taken place.
Ignatius of Antioch (circa 50 - 110AD)
Smyrneans:
They abstain from the Eucharist and from prayer, because they confess not the Eucharist to be the flesh of our Saviour Jesus Christ, which suffered for our sins, and which the Father, of His goodness, raised up again.
Philadelphians:
Take heed, then, to have but one Eucharist. For there is one flesh of our Lord Jesus Christ, and one cup to [show forth ] the unity of His blood; one altar; as there is one bishop, along with the presbytery and deacons, my fellow-servants: that so, whatsoever you do, you may do it according to [the will of] God.
Magnesians 9:
If, therefore, those who were brought up in the ancient order of things have come to the possession of a new hope, no longer observing the Sabbath, but living in the observance of the Lord's Day, on which also our life has sprung up again by Him and by His death— whom some deny, by which mystery we have obtained faith, and therefore endure, that we may be found the disciples of Jesus Christ, our only Master
Pliny the Younger, a Roman persecutor described the Christian worship (circa 112 AD)
...They declared that the sum total of their guilt or error amounted to no more than this: they had met regularly before dawn on a fixed day to chant verses alternately among themselves in honor of Christ as if to a god, and also to bind themselves by oath, not for any criminal purpose, but to abstain from theft, robbery and adultery, to commit no breach of trust and not to refuse to return a deposit upon demand. After this ceremony it had been their custom to disperse and later to take food of an ordinary harmless kind.
("to bind themselves by oath": in Greek, oath is sacramentum, the root of our word sacrament. The seven Sacraments are covenental oaths with which God blesses us.)
References:
Early Christian Writings
http://www.newadvent.org/fathers/
Topical quotes from the Early Christian Writings
http://www.staycatholic.com/early_church_fathers.htm
Fordham's Ancient History Sourcebook: Christianity
http://www.fordham.edu/Halsall/ancient/asbook11.asp
"Setting your house in order"
http://blog.adw.org/2014/04/set-your-house-in-order-in-four-easy-steps/
