SQL Server - Get the Last day of a given week

So how do we find the last day of the week for a given date?

-------------------------------------------------
-- Does the function already exist?
-- if so... drop it.
-------------------------------------------------
IF OBJECT_ID(N'dbo.xGetLastDayOfWeek',N'FN') IS NOT NULL
BEGIN
    print 'Function found... Dropping'
    DROP FUNCTION dbo.xGetLastDayOfWeek
END

GO

CREATE FUNCTION dbo.xGetLastDayOfWeek(@Date DateTime)
RETURNS DateTime
AS
-------------------------------------------------
--Function will return the last day of the
--week based upon the date passed in.
--
--You pass in a Date and it return
--the Date for last day of the week
-------------------------------------------------


BEGIN

    DECLARE @DayOfWeek      INT
    DECLARE @DaysToAdd      INT

    -- Get the day of the week
    -- i.e. if today is Thursday and the week starts on Sunday
    --      then the day of the week is 5
    SET @DayOfWeek = DatePart( dw, @Date )
   
   
    -- We need to subtract the @DayOfWeek from 7.
    -- This will tell us how many days we need to add to the
    -- current Date to find the end of the week.
    SET @DaysToAdd = 7 - @DayOfWeek


    -- Now we add @@DaysToAdd to the @date passed in.
    -- This will return the last day of the week for the given @date.
    RETURN DATEADD( d, @DaysToAdd, @Date )


    -- Instead of the previous five lines of code we could have also
    -- used this single line.  I broke it out above to hopefully better
    -- explain what was happening.

    --RETURN DATEADD(d,7-(DATEPART(dw, @Date) ),@Date)
END

GO 

-------------------------------------------------
--Now we will check to see if the code works
-------------------------------------------------

 
-- Should return 12/03/2011
Select dbo.xGetLastDayOfWeek('2011-12-01')


-- Should return 12/31/2011  
Select dbo.xGetLastDayOfWeek('2011-12-25')

 
-- Should return 12/31/2011 
Select dbo.xGetLastDayOfWeek('2011-12-31')