You left out a common possibility: Saturday. Much of the middle east/north Africa begin the week on Saturday.
Friday is a minute possibility, also.
https://en.wikipedia.org/wiki/Week#/media/File:First_Day_of_Week_World_Map.svg
(My week starts on Sunday, where I live, they consider Monday the start of the week. I know this, but internally, I've dealt with Sunday for most of my life and therefore...
but yes. It's not a Sunday/Monday thing. It's a Saturday/Sunday/Monday thing, with a slight chance of Friday.)