It's much easier to visualize with data.

Try this for example:

DECLARE @test-2 TABLE(myDate DATE, Sales MONEY)

INSERT INTO @test-2(myDate, Sales)

VALUES

('12/30/2013',100),

('12/31/2013',100),

('01/01/2014',100),

('01/02/2014',100),

('01/03/2014',200),

('01/04/2014',200),

('01/05/2014',100),

('01/06/2014',100),

('01/07/2014',100),

('01/08/2014',100),

('01/09/2014',100),

('01/10/2014',250),

('01/11/2014',250),

('01/01/2015',100),

('01/02/2015',150),

('01/03/2015',150),

('01/04/2015',100),

('01/05/2015',100),

('01/06/2015',100),

('01/07/2015',100),

('01/08/2015',100),

('01/09/2015',300),

('01/10/2015',300)

SELECT *, DATEPART(dw, myDate) AS Day_Week, DATEPART(isowk, myDate) AS Week_Num FROM @test-2

SELECT DATEPART(isowk, myDate) AS Week_Num, DATEPART(yyyy, myDate) AS [Year], SUM(Sales) AS TotalSales

FROM @test-2

WHERE DATEPART(dw, myDate) IN (6,7) --Friday and Saturday Only

GROUP BY DATEPART(isowk, myDate), DATEPART(yyyy, myDate)

You can use whatever predicate you want based on your needs. I just used this example to demonstrate grouping up Fridays and Saturdays over multiple years.