Another method I use, which might be of use...
By default using DATEPART/DATETRUNC on week in Tableau starts the week on a Monday (at midnight). If I want to report by week, and our client week starts on, say Sunday, I create a calculated field in the form
If you look at the below screen shot (afraid my oldest Tableau installation is 8.3, so won't be able to post you workbook you can open) you'll see the effect this has
The Date Time Week Default calculation is
and the SR version is the above formula
In your very exact case you could use
you may need to check the -15 is correct , but if you have a play this is one method.