The biggest factor in your calculations on when to visit Beijing should be the movement of domestic tourists, who during the longer public holidays take to the road in tens or even hundreds of millions, flooding transportation, booking out hotels, and turning even the quieter tourist sights into litter-strewn bedlam.
Peak Travel Seasons
Chinese New Year (Spring Festival) -- Like many Chinese festivals, this one operates on the lunar calendar. Solar equivalents for the next few years are February 14, 2010; February 3, 2011; January 23, 2012; and February 10, 2013. The effects of this holiday are felt from 2 weeks before the date until 2 weeks after, when anyone who's away from home attempts to get back, including an estimated 225 million migrant workers. If you are flying from overseas to Beijing, this won't affect you, but a land approach may be difficult, except in the few days immediately surrounding the holiday. Banks, as well as smaller restaurants and businesses, may be shut for a week. But main attractions are mostly open.
Labor Day & National Day -- In a policy known as "holiday economics," the May 1 and October 1 holidays have been reduced to 3 days each (including 1 weekend -- most people are expected to work through the weekend prior to the holiday in exchange for 2 weekdays, which are added to the official 3 days of holiday). These two holidays now mark the beginning and end of the domestic travel season, and mark the twin peaks of leisure travel, with the remainder of May, early June, and September also busy. The exact dates of each holiday are not announced until around 2 weeks before each takes place.
For the best weather, visit Beijing in September or October when warm, dry, sunny days with clear skies and pleasantly cool evenings are the norm. The second-best time is spring, late March to mid-May, when winds blow away the pollution but also sometimes bring clouds of scouring sand for a day or two, turning the sky a livid yellow. Winters can be bitter, but the city is much improved visually under a fresh blanket of snow: The gaudy colors of the Forbidden City's palaces are emphasized, as is the Great Wall's bleakness. Summers are humid and hot, but air-conditioning makes them tolerable. The number of foreign visitors is high during summer, but the Chinese themselves mostly wait until the weather cools before traveling.
A few years ago the Chinese were finally granted a 2-day weekend, but while offices close, shops, restaurants, post offices, transportation, and sights all operate the same services 7 days a week. Most sights, shops, and restaurants are open on public holidays, too, but offices and anything government-related close for as much time as possible. Although China switched to the Gregorian calendar in 1911, some public holidays (and many festivals) are on a lunar cycle, with solar dates varying from year to year. Holidays are New Year's Day (Jan 1), Spring Festival (Chinese New Year's day and the following 2 days -- see "Peak Travel Seasons" above, for exact dates in coming years), Labor Day (May 1 plus up to 4 more weekdays and a weekend), and National Day (Oct 1 plus extra days, as for Labor Day).
Note: This information was accurate when it was published, but can change without notice. Please be sure to confirm all rates and details directly with the companies in question before planning your trip.