Based on the original post, I'm thinking you want something like this:
declare @TestDate date = '20120101',
@DaysToAdd int = 8;
select
@TestDate,
@DaysToAdd,
dateadd(dd, @DaysToAdd, @TestDate),
datepart(wk,dateadd(dd, @DaysToAdd, @TestDate)),
dateadd(dd, datepart(wk,dateadd(dd, @DaysToAdd, @TestDate)) - 1, dateadd(dd, @DaysToAdd, @TestDate));