You can make priority seating arrangements up to 2 months in advance (so for Aug 16, you could make reservations Jun 16, etc).
(714) 781-3463 (option 4)
You should make PS arrangements for Blue Bayou if you want to eat there. Most restaurants don't fill up way in advance. I have called before 3pm for same-day dinner arrangements. However, if you go at a busy time, or on the weekends, I recommend making these arrangements a few weeks in advance.
In the parks you can make them for Blue Bayou, Big Thunder Ranch, & Cafe Orleans (Carnation Cafe is currently closed). All character meals take them, as well as Downtown Disney and hotel restaurants.