The best time to visit Beijing is spring or early fall; the weather is better and crowds are a bit thinner. Book at least one month in advance during these times of year. In winter Beijing's Forbidden City and Summer Palace can look fantastical and majestic, especially when their traditional tiled roofs are covered with a light dusting of snow and there are few tourists.
The weather in Beijing is at its best in September and October, with a good chance of sunny days and mild temperatures. Winters are very cold, but it seldom snows. Some restaurants may be poorly heated, so bring a warm sweater. Late April through June is lovely. In July the days are hot and excruciatingly humid with a good chance of rain. Spring is also the time of year for Beijing's famous dust storms. Pollution is an issue year-round but particularly in winter months.
Avoid travel during Chinese New Year and National Day. Millions of Chinese travel during these weeks, making it difficult to book hotels, tours, and transportation. If you must visit during Chinese New Year, be sure to check out the traditional temple fairs that take place at religious sites around the city.