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:
Hello Community, Today we are going to talk about the three Scrum Roles. There is the Development Team, the Scrum Master and the Product Owner. In my opinion these three are all really impo...
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