When creating an ad-hoc report using weekly periods, the data warehouse is starting on Monday.
However, in Modern UX as well as fiscal periods, the weekly dates start on Sunday.
Why is there a difference?
This may be caused by the Weekly Data Warehouse slices and the First Day of the Week are set to Monday, where the fiscal weekly periods start on Sunday.
If confirmed that the First Day of the Week can be changed do the following:
Now the Week Start will be Sunday for and ad-hoc or reports.
The only caveat here is that in Classic, the weekly TSV always start on Monday.