Generally speaking, the southern part of China is warm, humid, and rainy while the northern part is dry and windy.
In Spring (March to May) and Autumn (September to November) you will need a lined jacket or woolen sweater over light clothes. In Summer (June to August) cool cotton garments, such as T-shirts are recommended. In Winter (December to February) a light cotton-patted coat will keep you warm enough in the South, but in the North a heavy woolen coat is a must.
The best months to visit China are April, May, September and October, as all have a comfortable temperature.
Bare in mind, however, that the first week of October (1st to the 7th) is also the National Holiday and known as the golden week for tourism. During this period most major tourist attractions are always overcrowded with people and thus both hotel rates and airfares will hike their fares up quite a lot. So traveling in China during the National Holiday is not highly recommended.
