> I don't think anyone assumes Sunday as the first day.

That is not up to assumptions, but to conventions and arbitrariness. You find both conventions among Portuguese speakers, but mostly domingo/Sunday as the first day of the week.

A good way to check this is to websearch pictures for "calendário em português", and look for the first column of the calendar. Most will have D/Dom (domingo/Sunday); however a large minority will have S/Seg (segunda-feira/Monday) instead.

>And I leave the "market" for someone who still didn't have enough of this rabbit hole.

It's mostly a fossilised meaning.

Latin "feriae" originally meant holidays, holy days, vacations, and the likes; that's why the clergy¹ chose "feria" for the weekdays, as you'd be praising their god instead of the Pagan gods². However, already in Late Latin, this backformation also meant "fair"³. In Portuguese that second meaning eventually evolved to "street market", likely due to influence of the names of the days of the week - you don't work in domingo/Sunday or sábado/Saturday, but you gotta work and sell stuff in the "dias de feira" (feira days).

1. This clergy intervention was done in Latin. This is visible for the name of Tuesday being terça-feira ← tertia feria instead of *terceira feira (third holy day).

2. The traditional names for Monday to Friday were lues, martes, mércores, joves, vernes. Ecclesiastical interference got rid of the names for Sunday and Saturday early on, but if Portuguese inherited the Roman names they'd be something like "soes" and "sadurnes" respectively.

3. In fact that's where English got the word "fair" from. At least when it comes to those fancy markets/exhibitions/etc.; not to be confused with the homophone "fair" for just, beautiful, etc., as this one is native and unrelated.

Russian is similar to Portuguese, albeit off by one.

Monday = Segunda (second) = понедельник (start of the week)

Tuesday = Terça (third) = вторник (second)

Wednesday = Quarta (fourth) = среда (middle)

Thursday = Quinta (fifth) = Четверг (fourth)

Friday = Sexta (sixth) = пятница (fifth)

So I guess Russians have no doubt as to when the week starts.

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...

That sounds quite sensible. From memory, our (en_GB) weekdays are named like this:

Monday -> Moon day

Tuesday -> Tiw's day (Norse)

Wednesday -> Woden's day (Norse - Odin - chief god, one eye, two ravens)

Thursday -> Thor's day (Norse - god with a massive hammer) Friday -> Freya's day (Norse, rode a chariot drawn by cats)

Saturday -> Saturn's day (Roman, also: Saturnalia is the winter festival that eventually became Christmas)

Sunday -> Sun day

> That sounds quite sensible. From memory, our (en_GB) weekdays are named like this:

Spanish and English follow a similar day naming convention.

But the god's name needs to be translated from Norse Mitology to Roman Mitology first [1]

Lunes/Monday -> Moon/Luna

Martes/Tuesday -> Tiw/Mars

Miercoles/Wednesday -> Woden/Mercury

Jueves/Thursday -> Thunraz/Jupiter

Viernes/Friday -> Frig/Venus

Sábado/Saturday -> Saturn/Saturno

[1] https://en.wikipedia.org/wiki/Interpretatio_graeca

With the exception of Saturday. Tacitus associated the Hebrew sabbath («sábado») with Saturn, but the word for the day itself is not «saturno» as it would be if it were like English.

I'd always thought (though I don't have a specific source I can recall), that the seven days of the week was drawn from the seven non-fixed celestial bodies visible ot the naked eye: the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn. The planets were frequently associated with and named after deities, so the days of the week also have the names of deities.

Friday literally means Day of Frigg (Old English), referring to Frigga, the wife of Odin. Yes, Freya is believed to be the same goddess as Frigga, but the day is named after Frigga not Freya.

Thurday would probably be 'Þunor's Day' instead of Thor as it's Old English :)

They absolutely do in Portugal - I’ve been tripped up by “see you next Sunday” when they meant “see you this Sunday”.

> but I don't think anyone assumes Sunday as the first day.

You can get any calendar from a Portuguese speaking country to see they do.

Segunda-feria supposedly comes from Church Latin where it means 'second weekday'.