You should start by trying to change the data type of your field "WeekEndDate" into a date rather than a string.
In the data pane in the left of the workbook (where you have all your dimensions, the "blue pills") right click the dimension "WeekEndDate" > Change Data Type > Click Date. You can see now a little blue calendar on the left of the dimension.
By doing that you're telling Tableau to consider the dimension as a date, which will allow us to manipulate it much better.
If your data is updated regularly (up to today a a few days ago). We can also use the Date dimension as a much more precise filter. By dragging it onto the filter shelf > choose relative date > last week > click ok
Right click on the date filter you've just inserted > select show filter
The filter appears on the top right below "show me" by clicking on it you can change the week you're displaying in the workbook.
If it does not work correctly you can indeed use the date parse function.
This tableau article explains it well: Understanding the DATEPARSE Function | Tableau Software
Then using the function datediff can be useful.