We have been several times, the week before Christmas. Early in the week the crowd levels are great. Each day closer to Christmas, the crowd level builds. Year before last, we opted to avoid the parks completely on the 23rd because the crowds had gotten pretty crazy the day before. We chose to resort hop and enjoy all of the decorations instead. We had a wonderful time!
I agree with the previous poster. The weather is completely unpredictable. We have had beautiful mild weather, highs around 80 and freezing cold, winter weather- hats, gloves, coats,etc.!!!
Disney at Christmastime is spectacular! The parks and resorts are beautiful. The caste is extraordinary, candlelight processional is beautifully done and the osborne lights are great too! The holiday parades are fabulous!!! After the final Christmas Party at the Magic Kingdom, the holiday Wishes fireworks and holiday parade begin daily! It truly is a wonderful time to visit!
Have a great time!