My watch is displaying the week as MTWTFSS instead of SMTWTFS like everything else. What’s up with that?
You mean it used to do it the other way and suddenly changed, or this is the first time you’ve used an Apple watch? I’m not an Apple person so I don’t know what’s normal for that watch.
Starting the week with Monday is not that unusual, especially in business-related situations.
Fair enough, but I’m not a fan of M start. Su start for me!
For the OP, there’s probably a config setting for this.
Just got a new(er) phone and had a hell of a time pairing it up, so that may factor in somehow. I never noticed it before. I’ve never seen a calendar formatted like that, and I’m 74 of your Earth years old.
I’m a-lookin’.
I’ve seen it mostly on weekly presentations, like printed planners or week-at-a-glance type things. Lots of these things are still sold, to people who prefer writing to typing.
If you’re seeing it on a monthly calendar, I agree that is more unusual.
Monday first calendars are standard in some European countries. Every calendar app I’ve ever used has a setting for Sunday or Monday first.
If the OP can’t find a setting in the app, look in the settings related to time and date or language or region. I bet something is defaulting to not-USA style.
That was it! It was originally on “mirror my phone”, which seemed logical, but I went to Custom, and put in the US so that did it.
To get to the regular calendar I have to go through a calendar which I have no clue what it’s for. At the top are the days of the week, under which are their dates. The rest is vertical gray bars under each day, with colorful blocks of different sizes occupying those bars. It reminds me of a player piano.
The vertical axis is time of day. Each column is your activities in time order.
So it shows what you’re doing when.
On the iPhone there’s a calendar setting: “Start week on” which controls this behavior.
That’s definitely true for Germany. I was confused by the OP, but obviously judging by the answers, calendars starting the week with Sunday seem to be common in the US, which I wasn’t aware of. I’ve never seen a German calendar, printed or digital, that didn’t start with Monday. I thought that only the Christian churches insisted on Sunday being the first day of the week, while the whole business world uses Monday.
Where, exactly are you from?..& where do you live, Roswell, NM??? 
I’ve never understood how Sunday is both the end of the weekend but it’s also the first day of the week on a calendar.
I’ve already said too much.