I was sad to see someone engaging in a classical lexical fallacy over at the PuritanBoard (link). I don't know whether the moderators there will leave it up (they have been diligent in the past about eliminating misleading posts), so let me summarize the argument the person posting uses:
1) Acts 20:7 is translated in the KJV as saying that the disciples gathered on the "first day of the week."
2) Actually, the Greek word used is "sabbath" not "first day of the week".
3) Therefore, the disciples actually met on the Jewish sabbath, i.e. Saturday, which then spilled over into Sunday when the meeting went long.
End of the argument.
First of all, no lexical fallacy has zero basis, so what is the basis? The basis is the fact that Luke uses the word "σαββατων" (sabbaton) in the verse. Also, this word is frequently translated "sabbath" in the New Testament. These things are true, but the lexical fallacy is to stop there and say "aha, they met on the Jewish Sabbath, i.e. Saturday."
Luke doesn't just use the word "σαββατων" by itself he uses the expression "τη μια των σαββατων" (te mia ton sabbaton). What does this expression mean? It means "the first of the week."
It's the same expression used in John 20:19:
John 20:19 Then the same day at evening, being the first [day] of the week, when the doors were shut where the disciples were assembled for fear of the Jews, came Jesus and stood in the midst, and saith unto them, Peace be unto you.
It's the equivalent expression we see early in the same chapter
John 20:1 The first [day] of the week cometh Mary Magdalene early, when it was yet dark, unto the sepulchre, and seeth the stone taken away from the sepulchre.
in Mark 16:9
Mark 16:9 Now when Jesus was risen early the first [day] of the week, he appeared first to Mary Magdalene, out of whom he had cast seven devils.
and in 1 Corinthians 16:2
1 Corinthians 16:2 Upon the first [day] of the week let every one of you lay by him in store, as [God] hath prospered him, that there be no gatherings when I come.
But, you might say, that perhaps these are controversial texts. After all, maybe you want to believe that the word also means "Jewish Sabbath" in those passages too. Thankfully, God has provided a couple of more usages that show that this isn't so:
1 And when the sabbath was past, Mary Magdalene, and Mary the [mother] of James, and Salome, had bought sweet spices, that they might come and anoint him. 2 And very early in the morning the first [day] of the week, they came unto the sepulchre at the rising of the sun.
A form of the same word is used twice in those two verses: "σαββατου" in verse 1, when it says that the "sabbath was past" and "της μιας σαββατων" when it says that Mary Magdalene came "the first [day] of the week." I'm not sure it how it could be much more clear that this is the day after the Jewish sabbath, i.e. Sunday, the first day of the week.
But, in case, someone still doubts that the term usually translated "sabbath" can also mean "week" check out Luke 18:12:
Luke 18:12 I fast twice in the week, I give tithes of all that I possess.
Does someone really think that the guy means that fasts twice on the sabbath day as opposed to fasting two times in one week? That would be rather like quitting smoking two times in one day, don't you think? It's a ridiculous interpretation, just as the argument that the "τη μια των σαββατων" is not the first day of the week is a ridiculous argument relying on a lexical fallacy.
Oh, and how do I confirm that? Well, I confirm it by noting that "των σαββατων" is conjugated as a plural genitive σαββατων, not a singular one σαββατου. This is just one of a myriad of examples that I could give of people employing the lexical fallacy, where they get themselves a copy of Strong's concordance and figure that if the word usually means "x" it means "x" in this instance.
But, in these instances, Acts 20:7, 1 Corinthians 16:2, and the gospel accounts, we learn that the weekly (sabbath) meeting of the Christians was Sunday, the first day of the week, rather than Saturday, the Jewish sabbath.