STo find previous Saturday for given date
declare @selecteddate date = '20111118';
SELECT DATEADD(DAY, (DATEDIFF(DAY, '19000106', @selecteddate) - 1) / 7 * 7, '19000106') previous_saturday;
GO
To find Previous Sunday
declare @selecteddate date = '20111118';
SELECT DATEADD(DAY, DATEDIFF(DAY, '18991231', @selecteddate) / 7 * 7, '18991231')
GO