My code is not M code (power query language) it is a DAX formula to calculate a new column in your sales table.
1. Go to the "Data" tab (icon looks like a table in the menu in the left side) (Don't enter "Edit Queries")
2. Go to your table with your sales transactions
3. On the "Home" tab in the top menu select "New Column" under "Calculations" (just to the left of the Publish button)
4. Insert this code in the formula field "FirstDayOfWeek = DATE([Year];1;-2) - WEEKDAY(DATE([Year];1;3)) + [Week_number]*7" (and press enter)
You now have a new column in your sales table the should hold the first date of the week calculated from your year and week column and you should be able to make a relationship between this column and your date column in your calendar table.