Let me try to explain. Most of the time, you should be able to get what you want by waiting until 11 months ahead of your checkout date (last day of your proposed vacation) to get what you want.
However, let's suppose you want to book a GV at OKW for 5 days (Sun - Thur) next December. You want to check in on 12/7/03 and check out on 12/12/03. That is a popular time for
DVC members to visit WDW and there are a limited (small) number of GVs available.
If you wait until 11 months ahead of your 12/12 check out to book all at once, you might not get all of the days. In this situation, you would be better off to call on 1/8/03 to book the first day (12/7) and then on 1/9/03 to book the second day (12/8) and then again on 1/10/03 to book the third day (12/9) and etc. MS will link the reserations/days so that you end up with only one confirmation letter and one reservation number.
Remember, that reservations are booked on a first come, first served basis. Someone who calls day by day has a better chance of getting what he/she wants than a person who only calls once. That said, keep in mind that many members have reported successfully using the waitlist if they don't get what they want on the first try.
Most posters believe there are only certain size units and/or certain times of the year that calling day by day is really necessary.
If you want to be sure of getting an OKW GV on any weekday, a day by day strategy is considered necessary. Day by day ressies are also usually recommended for the summer weeks at HH (esp studios and 1 bedrooms) and all standard view rooms at BWV (anytime, any size). The first two weeks of December have become very popular at all the WDW DVC resorts and while it may not be absolutely necessary to call day by day, it certainly seems wise to call day by day for that as well. Many also recommend day by day for Easter, 4th of July and the weeks of Christmas and New Year's. IMHO, you absolutely must call 11 months ahead to get NYE at BWV (and probably now BCV).
HTH a little. I'm sure there are other posters out there who have other opinions/experiences.