When the Apostle John received the Revelation of Jesus Christ (Re 1:1), he tells us he was, “in the spirit on the Lord’s Day.” (10) This term, “the Lord’s day”, occurs only here in scripture, and it’s nearly universally understood by Christians to be a reference to Sunday, the first day of the week, though there’s no indication of this in the immediate context.
Why would Christians insist John is referring to Sunday? Typical reasoning is since the saints in Troas assembled for a meal and teaching on a Sunday (Ac 20:7), the early Christians must already have been observing Sunday as their day of rest and worship since Christ rose from the dead on Sunday. (Mk 16:9)
However, it is reasonable to think this particular meeting in Troas occurred in the evening, just after sunset on Sabbath, since Paul’s speech continued until midnight (speaking 5-6 hours straight is much more likely than 12-15 hours). The evening is when the new day begins, and it would be convenient for believers to meet just after sabbath on the first day of the week at or near a synagogue where both Jews and believing Gentiles would already be gathering to hear scripture read and expounded. (Ac 15:30)
Early believers only had the Tanach (Old Testament, there was no formal New Testament yet), and copies were very expensive; the synagogue likely had the only scripture in any given city. If one wanted regular access to the Word of God, synagogue was it. This is a very reasonable motivation for meeting on Sunday evening; we need not assume early Gentile believers were arbitrarily fabricating a new holy day to supplant the sabbath, breaking with their Jewish brothers and sisters to despise a basic command of God observed by saints for millennia.
Sunday was a workday for Jews, so it would have been problematic for any congregation with a sizeable Jewish element to assemble during the day on Sunday. There is no indication in scripture God told them to do so, there is no historical record of this practice during the apostolic era, and there is no clear motivation from their circumstances until quite late in the first century, so we may be confident they didn’t do this at first. The early disciples met daily, randomly, to eat and fellowship as much as they could, not just on Sundays (Ac 2:46), likely mostly informally in the evenings after work.
Paul’s instruction to Macedonian believers to allocate their alms for poor Jewish saints (Ro 15:26) on the first day of the week (1Co 16:2) is also offered as evidence that early believers were meeting on Sunday and setting it apart as holy. However, the text does not indicate this was a collection taken up in the assembly but dedicated privately at home. So, this text also does not indicate believers were setting aside Sunday as holy.
The above comprises the sum total of evidence from scripture suggesting Sunday is the Lord’s Day, and it’s inconclusive at best. Christianity’s insistence on Sunday must be driven by something other than scripture, by tradition starting well after the apostolic period as believers were desperately trying to distance themselves from Judaism and the burdensome Jewish Tax imposed by Rome. The eventual result was a new religion which was foreign to the apostles, corrupted from the true (Is 8:20), an imitation and counterfeit using all the same words and phrases, but fundamentally different, with new traditions and practices, often deeply antisemitic.
There’s only one reasonable choice for the Lord’s Day, it’s the day God Himself blessed and sanctified, the day He rested from His creative work: the seventh day. (Ge 2:4) This is the sabbath of the Lord thy God (Ex 20:10), that is … the Lord’s day.