Free dining has been offered to the general public during the month of September (very beginning and very end of the month may be excluded) every year since 2003.
While nothing is guaranteed, if you're traveling in September you may be able to plan on it.
Last year, the general public was offered free dining only in September and for certain dates in October, November and December (lots of blackout dates). For the later offer, several resorts were excluded entirely (Art of Animation, All Star Movies, Port Orleans French Quarter, Villas at Grand Floridian.)
If you're from the UK, you're in luck, there are free dining offers for UK residents for pretty much the entire year.
At times they also release PIN codes for free dining, but you must receive one of those to use it.