I don't think the week boundaries are all that important unless you map and / or measure your timekeeping in weekly units.
Having grown up in the USA, I was always accustomed to seeing Sunday as the first / leftmost day in the calendars' horizontal rows. This calendar display arrangement didn't mean much in my daily life, insofar as my timekeeping was based on specific dates and months.
As far as weekly blocks were concerned, the only subsidiary distinction I needed to recognize was 'work / school week' versus 'weekend'. The only start and end points anyone cared about were the ones delimiting these two intra-week blocks.
I've known a number of Americans who worked (e.g.) swing shifts that varied from week to week, but I don't recall anyone addressing the agendas and changes in terms of whole weeks rather than specific days.
Weekly-indexed timekeeping was an unexpected surprise when I moved to Stockholm to work for a government agency. The hyper-organized Swedish public sector folks prioritized weekly units in scheduling anything and everything. If anyone wanted to schedule a follow-on meeting, everyone would whip out their organizers and start comparing the opportunities in terms of (e.g.) Week 22 versus Week 23. It blew my mind, and it took some time for me to get accustomed to addressing everything in weekly increments.
I've studied and analyzed a number of workplaces and work processes that involved weekly changes back here in the States, but I can't say I've known anyone here to 'think in terms of whole week units', much less quibble about where those units' boundaries formally lie.