I hope this is right:
My translator found the same information that Erik Johnson did. There is no posted schedule for yearly operations. He said, “it is the perfect time to be there because it is not during the hot or the cold or the sand storms.” (Actual paraphrased quote of broken English) He felt that it should be open during the time that you are going to be there.
There seemed to be a daily schedule posted on the third tab under “News” http://bj.happyvalley.com.cn/news/index.asp This lists dates for one week but I’m not sure if it updates daily or weekly.
That is the best that I could do, not being able to read Chinese.