We went last January...2nd through the 6th, to be exact. The parks weren't crowded at all, and the E ticket night we attended was fairly deserted. Of course, part of that could have been due to the lousy weather. Basically for 2 out of the 5 days, it was in the low 40's and drizzly--- the other days were clear but COLD---til the last day when it finally hit the 60's, and of course, we had to leave. On E ticket night it was 32 degrees with a 23 degree windchill factor-- not something that happens often in Central Florida. The next morning, we were scraping ice from the seats of Aladdin before we could sit down to ride it! I don't think I have worn that many layers of clothing in my life. I felt like a "weeble that wobbles, but won't fall down"! Of course, the up side to the whole thing was that at home, they were having 12 inches of snow...lol! So, in perspective, we were having it GOOD!
Make sure to pack for ANYTHING, that way you don't go unprepared. There is nothing worse than going, and not having what you need to be comfortable!