International Labor Day falls on May first and depending on when it falls, workers might get and "extension". So, for example, in 2008, May 1 is a Thursday, so they'll get an extension on Friday. But it's not really a holiday because they have to go back to work on Sunday. Kind of illogical by some views, but it is, nonetheless, a few days off. every year, Chinese workers receive the week around May first off.
Travel during National Holidays:
Many workers will extend to make it a longer holiday which can translate into millions of Chinese traveling domestically and internationally. Travel fares double and triple and advance bookings must be made weeks, even months ahead for international travel. Hoards of tour groups flock to the major tourist destinations of China, so you can forget having a quite moment to ponder how the Great Wall was built.
May Holiday Travels:
If you can avoid it, it's advisable not to travel domestically during the week around May 1st. According to 2004 statistics, 90 million tourists were expected to travel; in 2006, China’s main tourist destinations saw a 17% increase in visitors. Four million tourists traveled to Shanghai alone.
But If You'll Be There Anyway...:
However, if you are in China, you'll find the weather in May is usually very nice, if a little wet. Government offices and banks will be closed May 1-3, but almost everything else, from tourist spots to shops, restaurants and the post office will be open for business.
gochina.about.com