It's August 25 to September 29. It's in effect right now for booking. If a guest is checking in between those dates and there is availability for your resort on the dates you want to stay there, there is no reason why you couldn't get the free dining. As noted, the availability is limited for each resort and each room category. For this offer, suites and the Art of Animation resort are not included at all.
As for someone reporting that they were told they couldn't get free dining, it could be that either the resort or room category they wanted was sold out of promotion rooms for their dates, their resort was not eligible, they wanted a booking outside the paratmeters of the free dining offer (for example, they were staying on a DVC reservation, they had annual passes or wanted to buy tickets separately) or they were checking in outside the promotion period. We would have to know the reason why the other person was turned down for free dining.
As the previous poster said, the best you can do is call Disney or use the online booking and see if free dining is available for the resort, room category and dates you want.
