Is this what you are looking for:

DECLARE @D DATETIME

SET @D = '2011-12-24' -- user passed value

SELECT SUM(Salesweek) FROM Sales WHERE Weekenddate BETWEEN DATEADD(ww,-3,@D) AND @D

Result: 9500

Now if the user passes in an improper date, for example

SET @D = '2011-12-23'

SELECT SUM(Salesweek) FROM Sales WHERE Weekenddate BETWEEN DATEADD(ww,-3,@D) AND @D

Result: 8500

Your T-SQL that executes the code, should check that the user passed value is a valid weekenddate. You could use an IF EXISTS statement to check that and return a message if the date is not valid.