Most people would think it took place on January 13, 1962, when CHEN, Huai-Shen, photographed the Shuan Cheng Tzu missile base. A few may argue it happened during the 1958 Taiwan Strait crisis. Unfortunately, both answers are not correct.
Based on what I have found in CIA declassified documents so far, the first documented overflight of China by a U-2 was on August 5, 1957. It was an Operation Soft Touch mission, originating from Pakistan.
Surprised?

U-2五十週年的布章
http://i45.photobucket.com/albums/f78/hch1990/501.jpg
That first overflight of China might had came from Lahore, Pakistan. 2 missions that day from Det B, B 4035 overfly USSR, while B 4036 over western ML China. The “B” might be designating as Detachment B’s missions.
http://www.spyflight.co.uk/u2.htm
You are absolutely correct. Operation Soft Touch missions were flown from Lahore, Pakistan, although it was not CIA’s first choice of operation location. The preferred site was Peshawar, whose runway was under repair.