If what I have postulated is correct, then there is no phrase for "first of the week" in the Koine Greek of the New Testament.
The best I have found so far is first, from
Luke 18:12. There, the Pharisee of the parable tells God in soliloquy that he fasts twice in the week. Here, "week" is the singular
sabbatou. Otherwise, I have not found the word "week" in the New Testament Scriptures outside of the other references already given (But this might just mean, especially as hyperbole, that the Pharisee claimed to fast two separate meals on the Sabbath to prove how devout he was).
So, I checked the LXX, and I see, for example that in
Genesis 29:27-28, the word translated as "week" is έβδομα, which is an ordinal derived from the Greek word for seven, that is, ἑπτά. As such, it means "seventh".
In
Exodus 34:22, the Greek word for "weeks" is εβδομάδων, that is, a period of sevens, which makes sense, since Pentecost is a celebration that takes place seven weeks, or a group of seven days by seven days, or 49 days, the day after Unleavened Bread ends.
Leviticus 12:5 reads δις επτά ημέρας, or seven days twiced, that is, two weeks.
Numbers 28:26 has εβδομάδων, or period of sevens.
Deuteronomy 16:9 has επτά, or the number 7.
Deuteronomy 16:10 has εβδομάδων, or period of sevens.
Deuteronomy 16:16 has εβδομάδων, or period of sevens.
2 Chronicles 8:13 has εβδομάδων, or period of sevens. This reference is interesting, because the word σαββάτοις, or sabbaths is also present, thus showing a different rendering between sabbaths and weeks.
Jeremiah 5:24 has προστάγματος, which has nothing to do with weeks or days, but rather, the order of appointments.
Daniel 9:24-27 and 10:2-3 have εβδομάδες, or period of sevens.
In all cases, then, the LXX knows nothing using any reference to the sabbath as a metonym. So, did using sabbath as a metonym for the word week come into existence after the LXX but before the NT? I haven't yet found anything to suggest it. Again, it seems like a translators choice.
I see in Justin Martyr's Dialogue with Trypho, Chapter 41, he claims Jesus rose from the dead on what might appear to be Sunday.
See:
https://www.logoslibrary.org/justin/trypho/041.html
Apparently, in the Talmud it is suggested that there are no names to the days of the week for Jews in Hebrew, so each day of the week is understood in relation to the Sabbath, so, the first day from the Sabbath would be Sunday, the second day from the Sabbath would be Monday, the third day from the Sabbath would be Tuesday, and etc. But I haven't yet been able to verify that as of yet. If this is the case, it might confirm that "first day of the week/sabbath" was a colloquial idiom referring to Monday.
Now, how to otherwise say "first of the week"? It looks to me like you could say something close to πρώτο της εβδομάδας or
próto tis evdomádas or first of the seven/week, but I need some help with that, as I am not sure. Maybe another member can chime in???
Finally, as far as tradition goes, I would ask, where does the tradition come from and why or how did it develop? If the tradition developed over time from a misunderstanding of what sabbath/s was/were in view, that is, the sabbath/s mentioned in the Gospels were the high days, then the tradition developed in error and can be discarded. But that is only a guess, as I don't actually know how and when and where and why the tradition developed.