I have a Lumia 730, and I am unable to configure it so that the week starts with sunday. How can I configure this?
The latest update for the Calender app allows you to set the first day of week, as well as the first week of the year. Check the Store for the update.
For more information, see Windows Central.
I believe this is a known problem and there is an outstanding feature request for this.
In other words, you can't!