We went last year on a Friday (12/12), as well as on a Sunday (12/14). There was definitely a difference in the crowd levels. Some places on Friday night were mobbed (and we're used to mobs since we go every year in July). That said, we didn't have any trouble getting into shows and still had a wonderful time. We did the shows, parade, and character greetings on Friday and rode on Sunday. I'd do it again, but I'd still go two nights.
This is OT, but bring a warm coat. We're from the cold Northeast and we froze at night during our trip. The weather's very unpredictable in December. I had leather gloves on every night and wished I had a heavier coat (so did my fiance who's a former hockey player).