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