Hi, Julie!
If you're planning to visit Walt Disney World for 12 days or more within one year, then an
Annual Pass is definitely the way to go! In your case, you'll want to consider purchasing the
Premier Passport which would give you Passholder benefits and discounts at both Walt Disney World Resort and Disneyland Resort.
I would recommend waiting until you arrive in August to purchase your Passport. The only reason to consider purchasing in advance would be if Disney announces a ticket price increase. Historically, price increases have occured in the early summer. The most recent increase was June 3, 2012. However, you'll want to determine whether the price increase is significant enough to warrant purchasing your Passport in advance since it will become effective on the day it is purchased/redeemed. For example, if you were to purchase your Passport in June (before a price increase becomes effective), but won't be arriving until August, you will effectively "lose" two months of the Passport's value. Some basic number crunching should help determine which is the most cost-effective option for you - purchasing in advance or waiting to purchase upon arrival.
No worries, you will still be able to take advantage of any passholder discounts on resort rooms when booking your August trip to Walt Disney World. You can simply go ahead and book your resort room with a passholder discount. When you arrive on August 21st, you'll just need to purchase your Passport before checking in at your resort since you'll need to show the Passport to receive the discounted room rate. I have always found the Passholder discounts to be a greater savings than the room discounts offered to the general public.
I hope this helps, Julie! Please let me know if I can assist you further with your planning needs.
Kimberly