As China is a big country, the best time to visit varies. Beijing is pleasant in the spring and fall, while a bit warm in the summer and cold in the winter. Thus April, May, June, September and October are peak travel months.
The south, such as Guilin, the Yangtze River and Hong Kong are more of a tropical climate with a rainy season, cool season and hot season. Between April and October it is most green.
Tibet is best in the summer (June, July August) when its warm but are also cool and crisp in the spring and fall with clear blue skies.
You need to be aware of national holidays and avoid traveling at these times if possible. The main holidays are Chinese New Year (Jan/Feb), May Day (week around May 1) and National Day (week around October 1). There are other one day holidays to be aware of but are not as much of an impact.
If you are in China over a holiday then taking a plane or train is similar to traveling around Christmas time in the west. It will be crowded and expensive, if you can even get a ticket. If you are traveling over national holidays then work with your travel agent to plan your itinerary to avoid peak travel days. Advance booking is essential and plane travel will be necessary as train tickets will be nearly impossible to get.
Major tourist sights are not closed over national holidays. Tour guides still work too. Some services may have a holiday surcharge as many workers receive triple wages as per Chinese labor law.
We still run normal tours through the holidays and don't charge extra. If you take a tour over a national holiday you should consider tipping a bit extra since your guide and driver have given up time with their families to serve you.