You accumulate points on anything you tag to, regardless of the month it was started in. The basic idea, though, is that only points accrued within the AC month will be able to be used for AC. The rest can just be used for wishes.

So, let's use your example.

July > 30 comments = 20 points (10 + 5 + 5) to use toward July AC
August > 65 comments = 15 points (5 + 5 + 5) to use toward August AC
September > 69 comments = 0 points
October > 72 comments = 5 points to use toward October AC

Does this make sense?