> 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.