It's not a given. It isn't something that WDW necessarily will do this year, although it's likely. It's up to them.
The "window" for free dining last year was smaller than previous years. Last year it was something like Aug 29-Sept 30. You could not extend the free dining into October by checking in prior to Sept 30, as you could the previous years. The free dining last year ENDED on Sept 30, so it was good for stays where the checkout date was no later than 9/30.
If you book for sometime in the middle of September, you'll have a good chance of getting free dining. If it's going to happen, you'll hear about it around June.
Actually last year free dining ended on Sept. 22. I know it ended Sept. 22 because several in our Grand Gathering were originally going to check in on Sept. 23 and they had to come in one day earlier to get the free dining. It carried through to the end of our stay on Sept. 30th.