> which is a totally separate question from the layout of a printed calendar

I have no strong preference on Monday/Sunday, and I don't think either is inherently more logical than the other. However, it would be illogical, whether Sunday or Monday first, to have the 7th and last day of the week, laid out as the first column on the calendar.

I like to have my two weekend days next to each other since they are a fairly discrete chunk of time in my mind and planning.

How would the colloquial understanding of the first day of the week get divorced from what's printed on calendars, though? I feel like one should inform the other, no?

Out of curiosity, if you have a calendar app on your computer, does it start with Sunday or Monday?

Majority of calendars in Japan are starts from Sunday but IMO now people think week starts from Monday. It's relevant to whether people work (or went to school) on Saturday or not (so two consecutive holidays). Some research shows that old ages (who worked on Saturday) tend to think it starts from Sunday.

Calendars tend to follow traditions, and the difference isn't much matter. Also computer apps are tend to follow the US culture since US dominates the industry.

I would guess that in a globalized world, stuff printed in the united states doesn't always match local culture.