What is the best month to visit Summer Palace

Autumn is the best time to visit Beijing and every attractive spots are worth seeing during Autumn.
If you want to see the leaves changing their color, I think Fragrant Hills Park is the best place. Mid October to the early November is the best period to visit the park, because most of the leaves at that time are changing their colors. It is wonderful.
I hope you can have a very pleasant trip in Beijing in this autumn.

Oct. may be the best time for traveling, and you can also go there in late August to September to experience its cooling surroundings.
Yeah, autumn can be good for traveling in Summer Palace. So don't worry about that . And you can certainly enjoy the colorful leaves then.