As with any major city, Beijing can be visited at any time of year as it has plenty of attractions that are available all the year round and don't depend on weather. However, from the weather point of view September and October are among the best months. The spring is short and there is a tendency to sandstorms, which puts a lot of people off, while the summer is very hot and humid, with a lot of rain in August. The city can also be rather busy in the summer because of school holidays.

Winter is cold, of course, but for a city break the winter is a possible time as there are good bargain breaks on offer and Beijing should be less crowded - though of course, like all Chinese cities it will be very crowded for New Year in February. Generally, you may want to plan your trip away from the major public holidays for this reason.

You can learn more about Beijing here.