Autumn is a good time to the Walls, when it is neither to hot nor too cold. There isn't too much rain either.
Late October to early November is another good time, for the leaves around Badaling Great Wall turns red and the scenery is so beautiful, but a little cold.
Walls covered by snow in winter time is also beautiful, but it will be chilly on the wall and one must keep warm.
just some personal thinking and you make the decision~