Compare:

DECLARE @date DATE = '20121202';

SELECT

CASE WHEN datename(dw,@date) <> 'Sunday' THEN CONVERT(varchar(10), DATEADD(WK, DATEDIFF(WK, 0, @date), 0),

103) ELSE CONVERT(varchar(10), DATEADD(WK, DATEDIFF(WK, 0, @date), - 7), 103) END

go

DECLARE @date DATE = '20121203';

SELECT

CASE WHEN datename(dw,@date) <> 'Sunday' THEN CONVERT(varchar(10), DATEADD(WK, DATEDIFF(WK, 0, @date), 0),

103) ELSE CONVERT(varchar(10), DATEADD(WK, DATEDIFF(WK, 0, @date), - 7), 103) END

go

DECLARE @date DATE = '20121202';

select dateadd(wk, datediff(wk, 0, DATEADD(dd,-1,@Date)), 0);

GO

DECLARE @date DATE = '20121203';

select dateadd(wk, datediff(wk, 0, DATEADD(dd,-1,@Date)), 0);

GO