If you want to cruise in Aug. of this year, I'd get looking around now. Do you have an idea of where and on what ship you want to cruise?
If so, you may want to start by posting in the "bargain finder" here at Cruisemates, great way to get prices and cabin availability now:
Once you post there, travel agents will e-mail you with what they have, based on the information your provide in your post. Also be sure to list the departure city if you want cruise/air included.
Good luck, hope you find just the right cruise to celebrate your anniversary!