Day used for beginning of the week

In forecast view on my laptop the week begins on Monday. In the same view on my desktop the week begins on Sunday.

How can I change things so both versions of OF2 have the week starting on Monday?

The especially confusing part is: (1) I’m seeing no obvious way to do this, (2) both Calendars and BusyCal start their weeks on Mondays on both computers. So it does not seem to be set by them.

I moved 3 posts to an existing topic: Calendar trouble back again [Solution inside]