This is one way.
DECLARE @TicketStartDate datetime
DECLARE @TicketEndDate datetime
SET @TicketEndDate = DATEADD(DAY, DATEDIFF(DAY, -1, GETDATE()) / 7 * 7, -1)
SET @TicketStartDate = DATEADD(DAY, -8, @TicketEndDate)
PRINT @TicketStartDate
PRINT @TicketEndDate
Also search this site for 'calendar tables'. Probably overkill for this situation but the method is great for date logic.