I've gone the week before Christmas several times, and my experience has been that the closer it gets to Christmas and New Years, the busier it gets. Once schools are out for Christmas break, it gets very busy -- 90 minutes for some rides, but I've always been able to avoid long waits by going early and using FP wisely. The 20th may not be too awful. Some schools may not let out for Christmas break until the 22nd. (My DS's school for example will be off the week of Christmas and the week after this year; typically they have more time before Christmas off, but not this year. I don't know if many other schools will be working on a similar schedule with Christmas falling on a Monday this year).