Hello Sonny,
You may use booing rules for this purpose. Under Bookings ->Booking Rules make sure to set the check-in and check-out days to Saturday. If you would like to restrict the maximum stay as well, set the max-stay option to 7 nights, so your guests could only book a cruise for 1 specific week.
The above configuration will allow your guests to book any desired week per their preference.
Regards,
Andre