1. Jesus made congregational worship his custom.
And he came to Nazareth, where he had been brought up. And as was his custom, he went to the synagogue on the Sabbath day, and he stood up to read.
(Luke 4:16 ESV)
2. Jesus Christ was resurrected on the first day of the week, Sunday.
Now on the first day of the week Mary Magdalene came to the tomb early, while it was still dark, and saw that(B) the stone had been taken away from the tomb.
(John 20:1 ESV)
3. On the first day of the week, Sunday, the disciples were assembled together, probably to commemorate Christ's resurrection.
On the evening of that day, the first day of the week, the doors being locked where the disciples were for fear of the Jews, Jesus came and stood among them and said to them, "Peace be with you."
(John 20:19 ESV)
4. One week later, the disciples were again assembled together.
Eight days later, his disciples were inside again, and Thomas was with them. Although the doors were locked, Jesus came and stood among them and said, "Peace be with you."
(John 20:26 ESV)
5. The book of Acts also describes how on another occasion the disciples were assembled together on the first day of the week, Sunday. The meeting included the breaking of bread (Communion, the Lord's Supper) and a sermon by Paul.
On the first day of the week, when we were gathered together to break bread, Paul talked with them, intending to depart on the next day, and he prolonged his speech until midnight.
(Acts 20:7 ESV)
6. Paul's Letters confirm the same thing.
On the first day of every week, each of you is to put something aside and store it up, as he may prosper, so that there will be no collecting when I come.
(1 Corinthians 16:2 ESV)
7. John called Sunday, the Lord's day.
I was in the Spirit on the Lord's day
(Revelation 1:10-11 ESV)
(Note: This Greek phrase is completely different from "The Day of the Lord")
8. Besides all this, congregating is a command.
And let us be concerned about one another in order to promote love and good works, not staying away from our worship meetings, as some habitually do, but encouraging each other, and all the more as you see the day drawing near.
(Hebrews 10:24-25 HCSB)
9. Early Christian and Non-Chrsitian writers confirm the above points.
The Didache - (AD 70-120) Chapter 14. Christian Assembly on the Lord's Day
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: A.D. 140
“But Sunday is the day which we all hold our common assembly, because Jesus Christ, our Saviour, on the same day rose from the dead.” Apology, Chapter LXVII.
Anatolius, A.D. 270, Bishop of Laodicea, in Asia Minor:
“Our regard for the Lord’s resurrection which took place on the Lord’s Day will lead us to celebrate it.” Chapter X.
Cyprian, A.D. 250, Bishop of Carthage in Africa:
“The eighth day, that is, the first day after the Sabbath and the Lord’s Day.” Epistle 58, section 4.
Tertullian, A.D. 200, in Africa:
“We solemnize the day after Saturday in contradiction to those who call this day their Sabbath.” Apology, Chapter XVI.
A.D. 112, Pliny, governor of Bithynia (central-northern Turkey), explains the elements of their subversive worship:
(1) Hymns about Jesus sung as part of early Christian worship; (2) prayer to God "through" Jesus and "in Jesus' name," and even direct prayer to Jesus himself, including particularly the invocation of Jesus in the corporate worship setting; (3) "calling upon the name of Jesus," particularly in Christian baptism and in healing and exorcism; (4) the Christian common meal enacted as a sacred meal where the risen Jesus presides as "Lord" of the gathered community; (5) the practice of ritually "confessing" Jesus in the context of Christian worship; and (6) Christian prophecy as oracles of the risen Jesus, and the Holy Spirit of prophecy understood as also the Spirit of Jesus. (Pliny (the Younger), Epistles in J. Stevenson, ed., A New Eusebius: Documents Illustrative of the History of the Church to A.D. 337 (London: SPCK, 1974), 13-15.)