We are going the last week in December and have already booked reservations for six of the seven nights we are going to be there. Two restaurants (Spago and Lahaina Grill ) are already booked in the peak time periods (6:00-8:30) periods for the nights we are going I found out later. I would go ahead and book your times, you can always cancel.
I'm heading over for mid-Dec, and am making all reservations now. Normally, I'm about a month out, but had to work out some schedules with friends joining us from O`ahu.
Now, I have heard conflicting reports on the level of tourism in the Islands. I do know that many travel entities are contacting me with reduced fares and rates. There might not be that much traffic. OTOH, if these promos work, there might well be more than normal. Who knows?
Still, for fine-dining, I like to have it all handled 4-6 weeks out and be done with it. I can also give my house-sitter the phone number, times, etc. of all of our evening meals.
I'll try to get a quick report posted, as soon as I return. Do not know if I'll make it up in time for your trip, or if my reviews are of any help.