The following function returns Monday's date for any input date:

CREATE FUNCTION fnMondayOfWeek (@AnyDate datetime)
RETURNS datetime AS
BEGIN
return dateadd(yyyy, datepart(yyyy,
dateadd(weekday,1-datepart(weekday, @AnyDate),@AnyDate))-1900, 0)
+ dateadd(dy, datepart(dy, dateadd(weekday,1-datepart(weekday, @AnyDate),@AnyDate)),0)
END

-- SELECT dbo.fnMondayOfWeek ('2006-05-12T00:00:00.000')