Is "First day of the week" an idiom?
By definition and usage it can't be.

An idiom, by definition, can vary in usage only a very little or its meaning is not understood. (See Part 1, idioms and their meaning for more about "first day of the week" as an idiom.)

"He put some wood on the fire" does not work, idiomatically, the same way as "adding fuel to the fire." The phrase "first day of the week" used in the New Testament cannot be a correct translation - since the original words do not mean that - unless it is an idiomatic expression. However, even that is greatly suspect as shown below.

In Mark 16:

"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." (Mark 16:9)

we have the same phrase in the English - "first day of the week" - that is elsewhere translated from "mia ton sabbaton." However, in this one case, it is translated from the Greek "protos sabbaton," literally, "first sabbath."

What we would then have is two different original phrases - "mia ton sabbaton" and "protos sabbaton" being translated into the same "first day of the week" with the understanding of an idiomatic meaning. Surely, this cannot be; it does not fit the idea of idiomatic usage at all.

Because "protos sabbaton," cannot be what translators so desperately want it to be, the two ancient manuscripts perhaps most closely connected with the Catholic church (Codex Sinaiticus, and Codex Vaticanus) have just omitted it altogether along with the rest of the verses of Mark 16. It appears in most other manuscripts and, logically, should be there.

It does not make sense that Mark would end his book with verse 8. The women were told to go and tell the disciples that Jesus had risen. In this verse, they simply told no man on the way to tell the disciples. They were going to do what they were told as quickly as they could while avoiding distractions. If Mark ended his gospel with verse 8, then he would have the women doing the opposite of what it says they did in the three other gospels (Matt 28:8, Luke 24:9, John 20:18).

If "first day of the week" from "mia ton sabbaton" is an idiomatic reference to Sunday it would be expected to appear in other literature but it doesn't.

Note: The next two paragraphs are from the article "Is the phrase 'first day of the week' properly translated in the New Testament?" by Daniel Gregg which appears at www.torahtimes.org/Sabbaton_Week_Sabbaths.html. (Emphasis is mine.)

"Outside of eight texts in the NT (Matthew 28:1; Mark 16:2, 9; Luke 24:1; John 20:1, 19; Acts 20:7, and 1Cor. 16:2), where we find μια των σαββατων translated as first day of the week, there is no example of σαββατων having the meaning of "week" in any Greek literature before ca. AD 100, and then only in "Church" Greek after that. ... This sense is entirely wanting in Secular Greek, the LXX, Josephus, Philo, or any other Greek literature of Jewish provenance before the destruction of the Second Temple except for these eight texts. That sense is also entirely lacking in classical and Koine Greek except for its alleged use in these eight texts. Furthermore, these eight texts are not just ordinary examples where nothing is at stake. What is at stake here is the original separation of Christianity from its Jewish roots, and the justifications supplied for this schism. Therefore, we may rightly suspect that the alleged sense of "first day of the week" is due to opportunistic revisionism based on sectarian religious and political motivations."

"There are several points that secure the above sense beyond reasonable doubt. First, according to Lev. 23:15 "seven Sabbaths" were actually counted following the Passover. If μια των σαββατων is counting the first of these seven Sabbaths, then we expect it to be used just after the Passover, and nowhere else. And this is exactly what we find. Yeshua was crucified just before the annual Sabbath; then the resurrection day just happens to be the first sabbath after the Passover, and it is called μια των σαββατων. In the other two uses in the NT, the same synchronization with the Passover is observed. Acts 20:6 tells us about the Passover before introducing the key phrase μια των σαββατων. In 1 Corinthians 16:2, it is mentioned that Pentecost is coming not too much after μια των σαββατων (cf. 16:8). Hence all of our texts fit the chronology implied in Leviticus 23:11-16 exactly.

Here is a list of resources (also from www.torahtimes.org) admitting that the original "mia ton sabbaton" was referring to Sabbath.

1. The Concordant Greek Text
2. The Concordant Literal New Testament
3. Robert Young's Literal Translation
4. Some of John Calvin's Commentaries
5. The Companion Bible
6. The Interlinear Bible by J.P. Green, 2nd Edition, Vol. IV
7. The Coverdale Quarto Bible, 1537
8. Some of Wycliffe's Translations
9. Some of Tyndale's Translations
10. The Rheims version, 1582
11. The Bishop's Bible, 1568
12. The Latin Vulgate
13. The Old Latin
14. All Greek Texts
15. The Syriac Peshitta

The definition and correct understanding of "mia ton sabbaton" depends on whether it is an idiom or not. This two-part study has helped provide a better understanding of the correct usage of "mia ton sabbaton" and the possibility of it having been used idiomatically.

This page (part 2) has shown that there is a lack of evidence that it has been used that way in early Christian literature. There is a distinct possibility that the claims made more recently (because of centuries of traditional understanding) for its idiomatic usage have been in support of Sunday sacredness.