I helped plan a trip about a year ago with
free dining dates early in December.
Late December early January is a VERY popular time so they never offer free dining then. My DBF and I go every September BECAUSE they usually offer free dining. Kids are back at school and it is the start of hurricane season so my guess is that they offer it to try and bring more people into the parks/resorts. I would suggest to wait until they offer free dining in September, should be probably before or around March as that would be 160 days or so out from the trip (so you could book dining reservations).
Free dining is always a good perk and to us (a couple) it is well worth it price wise. Some of the meals we have are upwards of $100 and when all you have to pay is gratuity it is definitely worth it to us.
Good luck and enjoy your trip no matter when you go!