The Christian practice of holding regular services on Sunday is so widespread that the few Christian denominations that observe the Jewish Sabbath, which falls on Saturday, are a distinct minority. Why do Christians worship corporately on Sunday rather than the Jewish Sabbath, Saturday?
The answer is simple and obvious. Christians gathered together on the first day of the week instead of the last day of the week because it was on that day that Jesus was raised from the dead. Christians were not bound by Jewish law to observe the Sabbath, so they gathered on what they called “the Lord’s Day.” Contrary to Dan Brown’s The Da Vinci Code, this had nothing to do with sun worship or pagan practice. Rather, it focused on the timing of the defining event of Christianity – Jesus’ resurrection. Also contrary to Dan Brown, this was not a later development imposed on Christianity by Constantine, but regular Christian practice hundreds of years before Constantine became emperor in 306 AD. Indeed, the practice of Christian worship on Sunday begins at the beginning. Acts and Paul himself records that early Christians were regularly meeting on the first day of the week.
On the first day of the week we came together to break bread. Paul spoke to the people and, because he intended to leave the next day, kept on talking until midnight.
On the first day of every week, each one of you should set aside a sum of money in keeping with his income, saving it up, so that when I come no collections will have to be made.
1 Corinthians 16:2
This practice soon became known as meeting on “the Lord’s Day.” Revelation is the earliest reference to this being called “the Lord’s Day.” It was on that day that John received his revelation, “On the Lord's Day I was in the Spirit, and I heard behind me a loud voice like a trumpet.” Revelation 1:10
Beyond the New Testament, widespread and early Christian writers attest that it was regular Christian practice to worship together on “the Lord’s Day,” the first day of the week. Many of these same writers explained why Christians did not honor the Jewish Sabbath.
The Didache, written in the latter part of the first or early second centuries AD, was a kind of manual for Church practice. It records how Christians gathered together “every Lord’s Day” for feasting and thanksgiving:
But every Lord’s Day, gather yourselves together, and break bread, and give thanksgiving after having confessed your transgressions, so that your sacrifice may be pure.
Didache 7.381. You can read the document for yourself, here.
Another very early Christian writing is the Letter of Barnabas, written around 100 AD. Although the author is unknown, he attests to Christian worship on Sunday, which he calls "the eighth day."
We keep the eighth day [Sunday] with joyfulness, the day also on which Jesus rose again from the dead.
Barnabas 15:6-8. You can read the document for yourself, here.
Next we have the testimony of Ignatius, a Christian leader of the first century AD. Ignatius wrote a number of letters around 110 AD while being transported to Rome to be executed. One of those letters, to the Magnesians, reports that Christians expressly rejected worshipping on the Jewish Sabbath, and instead chose to worship on “the Lord’s Day” because that was the day of Jesus’ resurrection.
If, therefore, they who were under the older dispensation came into a new hope, no longer keeping the Sabbath, but living in observance of the Lord's day, on which day also our life rose through him and through his death, which certain deny, through which mystery we have received faith (and through this abide, that we may be found disciples of Jesus Christ, our only teacher).
Letter to the Magnesians, 9.1. You can read the letter for yourself online, here.
Then there is Justin Martyr, a church leader in Rome who wrote around 150-60 AD. He explains at length the Christian practice of worship on Sunday and the reason for it:
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 rise together 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.
You can read Justin Martyr for yourself, here.
Tertullian seems to argue directly with Dan Brown. Tertullian was a Christian leader who wrote around 197 AD. He distinguishes Christian practice on Sunday from that of the pagans. He was well aware that some pagans also worshipped on Sunday:
Others . . . suppose that the sun is the god of the Christians, because it is well-known that we regard Sunday as a day of joy.
To the Nations 1: 133.
But Tertullian also knew that Christians had their own, non-pagan, reasons for worshipping on Sunday.
We devote Sunday to rejoicing for a far different reason than sun worship.
Apology Chap. 16.
Tertullian also explains that Christians do not practice the Jewish Sabbath:
It follows, accordingly, that, in so far as the abolition of carnal circumcision and of the old law is demonstrated as having been consummated at its specific times, so also the observance of the Sabbath is demonstrated to have been temporary.
An Answer to the Jews, Chap. 4. See also An Answer to the Jews, Chap. 2; On Idolatry, Chap. 14.
Many other early Christian leaders attest to Christian worship on Sunday rather than the Jewish Sabbath: Clement of Alexandria, 195 AD (speaking of “keeping the Lord’s Day” by “glorifying the Lord’s resurrection”); Origen, 248 AD (describing how Christians observe certain days, including “the Lord’s Day”); Anatolious, 270 AD (saying it should not be lawful to celebrate Jesus’ resurrection on any day “but on the Lord’s Day”); Victorinus, 280 AD (writing that Christians fast on Saturday but break bread and give thanksgiving on “the Lord’s Day”); Peter of Alexandria, 310 AD (“We Celebrate the Lord’s Day as a day of joy. For on it, He rose again”).
Clearly, therefore, the contemporary Christian practice of worshipping corporately on Sundays was enacted hundreds of years before Constantine, starting as it did with the early Christians. Moreover, the reason for the Christian practice of meeting on Sunday was to honor “the Lord’s Day,” the day of Jesus’ resurrection. It had nothing to do with pagan sun worship.