the calendar takes it's start/end days of the week from the local language pack.
if you want it to start on a Sunday, you will need to install the US pack and change your local user settings to US English. You will have to do this for every user so it's not a solution for a large install.
It's a common request, if you look at their Jira you will find:
Hey Community mates! Claire here from the Software Product Marketing team. We all know software development changes rapidly, and it's often tough to keep up. But from our research, we've found the h...
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