You book for the day you will be departing the resort and count back 7 months.
If you plan on going to BCV for 7 days departing 11/21/06, then you can book that 7 day vacation beginning today(under the 7 month window).
If you plan on visiting during an especially busy or popular time of year, then you may want to call MS every day and book that day. Otherwise, you could end up calling MS and find out that your last day is available(11/21), but the previous 6 days(11/15-11/20) are fully booked up. This is what happens with school vacation and Xmas weeks.