Lines are short, park hours are short, and weather can go either way. I've been there when it's been hot in December and cool in October. Our November trip was both! We needed light jackets in the evening, but it was definitely warm enough during the day to swim.
I would definitely go back in November. I liked October, but I didn't like missing the fall weather up north. I liked December, but I didn't like missing some of our Christmas preparation time at home. November has not much going on at home up until Thanksgiving. I voted for "It's a good time to go," but in retrospect I might say it's the best!