I highly doubt that they will wait until after July 17th to tell us anything
if the Halloween party is going to start in late September as usual,
OR if more parties are being added to the line-up in mid-September. The tickets would have to go on sale to AP holders/Disney Visa folks/DVC members by right around 7/17 or 7/24, at the very latest, and the general public tickets would follow within a week or two after that. They'd have to give some advance notice before the tickets went on sale -- at least a full week, if not more.
The only scenarios in which I see that kind of extreme delay in an announcement happening would be if:
1. The Halloween Party is not beginning until October; or
2. There are more big announcements coming about events happening on July 17th or during the week of the 17th; or
3. The Halloween Party is not happening at all.
Otherwise, July 17th comes and goes every year. The bigger deal this year was/is the overall Diamond Celebration and the entertainment that goes with it. And that is already well underway at this point. Unless Disney has some extra gigantic plans for July 17th that they need to start hyping next week, I can't see how it would be beneficial to wait until after 7/17 to announce party dates -- especially if more dates have been added in somewhere.
But, as I mentioned a couple of days ago, the Parks Blog may not even find out the info until a day or two before they have to write a blog about it. Who knows when the Team Disney Anaheim folks tell them anything?
Whenever Erin posts the blog about Halloween Time and Mickey's Halloween Party, you can bet that there will be some sort of sentence in it that says something like, "Even though it's summer, Halloween is just around the corner," or any variation of that message. In other words, they always have to 'justify' why they are making a Halloween announcement in July!