They don't necessarily open up the reservation times when the monthly hours are posted. They may open them up a month ahead, or a few weeks ahead, there's no set procedure, I'm afraid.
For instance, I wanted a ressie at Tony's Town Square the night they are running Spectromagic at 9pm. I want to sit on the porch (which they cannot gaurantee - the whole thing's a crap shoot. LOL!). Conceirge could only get me 5:45pm before the August hours were posted. Once the hours were posted, I asked them to change it, and they said they couldn't get anything later. I called Disney dining, and they also said they couldn't do anything later than 5:45.
I called back twice after that, and this past weekend, Disney dining changed my ressie to 7:30pm.
So, you may have to call a few times, but closer to your departure!