http://www.wingchina.com/en/flying%20the%20great%20wall%20of%20china.htm
This site is best for June to Oct.
Access
3 hours drive from Beijing, 120km. The Simatai Great Wall is a famour place to visit ,so nearl every local driver know how to go there.
WEATHER
Basically S,SE,SW wind can fly, and the take off site is at the half height of the mountain, so you should check the real wind direction