Hi, Catherine! I wish we had the answers regarding free dining - we'd be the most popular moms ever.
There is no guarantee of free dining, and we don't know before the public when it will be offered. You can do a Google search to see when it has historically been offered if you want to get an idea of when it might occur, but bear in mind that there aren't set dates for when or if free dining will be an available special offer.
You can usually bet that if they will offer free dining, it will be during traditionally less crowded seasons. Right after summer into September has been offered in the past, as has early December, before holiday crowds gear up. I have also seen it offered in January in the past.
Your best bet is to keep your eyes peeled at the official
Walt Disney World website. They list special offers often. I'll keep my fingers crossed for you!