I think except for small, really popular venues, it's not a problem. Why? Monday night, a mere 32 days before I arrive, I was able to get new ADRs and change around some of the ones I already have. And yes, at the same time I cancelled the ones I won't be using

in case that comes up as a peeve later in this thread. Regarding your December ADRs, I'd suggest calling every few weeks. I think they open more ADRs as the dates approach - example, when I originally tried to get Coral Reef, back in April, the latest dinner time available was 5:20; a few weeks later, I called and was able to get the same night at a
much more reasonable (for me) 8:30