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:
We are excited to announce the open beta program for self-hosted runners. Bitbucket Pipelines Runners is available to everyone. Please try it and let us know your feedback. If you have any issue...
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