On a side note, it's not 100% clear (to me, at least) that the word for Monday (ponedelnik) means "per week" or anything about the week per se.

In Russian the word for week is "nedelya", so "ponedelnik" does seem to naturally tie into that. But the word for Sunday in Russian is "voskresenie" (literally means resurrection). Notice this! Every religious word used in non-religious context is suspicious!

If we examine Bulgarian, the week is called "sedmica" (sedm == seven). And Sunday is called "nedelya" (I perceive it as "ne delya" === "no work").

Thus, in Bulgarian the word ponedelnik becomes like "the day after Sunday", rather than "the day starting every week".

Now going back to the Sunday resurrection theme, it's obviously of religious origins, so I dare assume that it may have replaced something that was already there (like nedelya). And then I imagine the people still wanted to keep the word around, so they started using it to mean "week" instead of "Sunday".

If so, then it would make sense that ponedelnik just means "post Sunday".

[The above is just a pet theory I haven't spent any time on, so I'm not guaranteeing its legitimacy]

This is correct, "nedelya" (no-work) is the common Slavic word for Sunday. The Russian variant is a more recent invention.

Note that this off-by-one is exactly the difference between Jewish/Biblical-based weeks, where the Sabbath is clearly the last day of the week (based on Genesis, the day when God rested), and more "revisionist" Jesus Christy-based weeks, where the day that Christ was resurrected is considered more important than the Sabbath and takes its place as the end of the week.

While the majority of Christians meet on Sunday and consider it the “new” Sabbath or the fulfillment of the Sabbath, in commemoration of Jesus’s resurrection, how does that relate to restructuring the week so Sunday is the last day? The Bible is clear that Jesus was resurrected on the first day of the week, the day after the Sabbath.

Matthew 28:1 (NIV): “After the Sabbath, at dawn on the first day of the week, Mary Magdalene and the other Mary went to look at the tomb.”

Mark 16:1–2 (NIV): “When the Sabbath was over, Mary Magdalene, Mary the mother of James, and Salome bought spices so that they might go to anoint Jesus’ body. 2 Very early on the first day of the week, just after sunrise, they were on their way to the tomb…”

Luke 24:1 (NIV): “On the first day of the week, very early in the morning, the women took the spices they had prepared and went to the tomb.”

John 20:1 (NIV): “Early on the first day of the week, while it was still dark, Mary Magdalene went to the tomb…”

I believe, though am not sure, that once Christians stopped observing the Jewish Sabbath and instead resting on the Lord's day, it also became natural to consider it the end of their week, instead of the beginning.

Then why does the US, a very young culture compared to Europe, universally consider Sunday the first day of the week? Calling Sunday the seventh day must be a very recent development.

The USA was founded and its culture heavily influenced by very deeply religious Protestants. While by the time of the founding of the USA, Sunday was decently well established as the seventh day in both Catholicism and Orthodoxy, most protestants were Bible originalists.

I believe that they resurrected the practice of considering Sunday as the first day of the week, based on the Old Testament tradition. Just like New Earth Creationism is in fact a relatively recent phenomenon driven by a renewed interest in literal interpretations of the Bible, that had long since been superseded in Catholic scholarship.

Actually Orthodoxy is arguably more inclined to treat Sunday as the first day of the week as witnessed by the Greek language.

There are also several patristic writings that conceptualise Sunday as the eighth day of creation, i.e. the resurrection perfecting and completing creation (this also suggests that literal seven day creation wasn’t taken that… er… literally back in the day).

Well, in Polish, Monday is poniedziałek, which means "after Sunday" (Sunday is "niedziela") and not "start of the week" and it looks analogous in Russian...