I've been twice during that time in the past 3 years. Certainly the 16th-20th will not be crowded. After that, the crowds begin to come in, increasing every day as it gets closer to Christmas. However, the parks are definitely not nearly as crowded as the week between Christmas and New Years Eve. The week before Christmas, it will not sell out.
Having said that, I would definitely keep reading these boards for good tips about how to handle crowds. Getting there early, using FP, and understanding which rides have long/slow moving lines and which don't are probably the 3 key tips to doing a lot during a crowded season. Also, the week before Christmas is great because the hours are normal "crowded park hours" (8AM-12AM) even though the crowd levels are not as high as other times. This means you can do a ton more AND the entertainment schedule is running in full swing (fireworks, fantasmic, Electric Parade nightly)