Hello!
I’m from a place where my brain is hard-wired to see the first day of the week as Monday, as Saturday and Sunday are the weekend, and so I can’t for the life of me get used to Sunday being the first day in a calendar.
I like the Heat Map widget, so I’d like to customize it to show the “correct” day as the first day of the week!
Haha, yeah, that was also the thing that was the hardest to get used to after I switched my smartphone and google account to Japanese. I still don’t get why Japanese start the week on Sundays 
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Good luck with getting your request granted.
Never thought about this before. For me the week starts on a Sunday. And naturally ends on a Saturday. I dont think there’s any difference since its the same week no matter how you slice it.
I could find a reason for Sunday calendars, but the reason for Monday starts isn’t as clear. Even the research papers are too murky and it devolves into asking why humans are the way they are. Just are.
Workweek starts on Monday?
ISO 8601?
The weekend shouldn’t also be the start of a week?
It just is?
Well, I’m the opposite, I can very easily find a way for Monday calendars, but the reason for having the week start at Sunday makes no sense to me. 
It is based on the workweek, indeed. That’s why we call Saturday and Sunday the weekend days, because they’re the days you get to yourself after you’re done with a normal daytime job. Even places that start their calendar at Sunday call Sunday a weekend day.
So like, Sunday is part of the END of the week, everyone everywhere agrees that Sunday is a weekend day. It makes no sense to me to then consider it the start of the new week, when it’s also the end of the old week. You’re never gonna experience a time that’s both Monday and Tuesday, as it can only be Tuesday because Monday has ended. Similarly, you will never experience being in both February and January at the same time. It can only be February because January has ended, it can only be 2026 because 2025 has ended. Because that’s how time works, it can only be a new time because the old time is in the past.
Weeks are the same way, a new one can only start because the previous one has ended. So Sunday can’t be both the new week and the old week, it has to pick one. Either it is the end of a week, or the start of a week, it can’t be part of two weeks any more than any period of time can be part of two days, two months or two years. I genuinely don’t understand how you find these reasons unclear and murky, how is it not completely logical and sensible to say a day can’t be part of two weeks at the same time? That it can’t be both the end and start of a week? 
But as you say, this is mostly just about what you’re used to. You internalize whatever system you’re used to, and make it feel like that’s just how it’s supposed to be. Which is why it’s very difficult for me to adapt to the calendar view that to me is kind of nonsensical. It’s easy for you to say it’s the same week no matter how you slice it, because you get the calendar the way you’re used to it. To me, it really isn’t the same at all 