I've been to Disneyworld at least 20 times, and only once Dec 26th to Jan 2. That is the busiest week of the year, and the parks will close as they fill to capacity, in stages. Staying off Disney property that particular week is tricky; there will be tons of traffic, and as the parks fill, they will begin with not admitting those not staying onsite. The time we did that, we stayed concierge level at a Disney resort, so we spent a great deal of time at the resort. We also had the concierge team book our dining and activites. So we'd go to a park for a couple of hours, then back to the resort and enjoy the amenities there. I really would agree with either waiting until after Jan 1st, or staying at a Disney resort, and planning for crowds like you've never seen.