The simplest solution would be to create a view that matches sale days each year up by weekday, and then use a percent difference table calculation. The first worksheet in my attached example worksheet show this solution.
The second option is use a FIXED level of detail (LOD) expression to fix the sales values to the week and weekday combination across years.
Directions for both methods are in the attached workbook.
I hope this answers your question!
Sample-1.twbx 1.1 MB