They are not celebrating Earth Hour in China; it conflicts with Serf Liberation Day, a holiday intended to "thoroughly reveal the vicious nature of the feudal serf system and the ulterior political purpose of the 1959 armed rebellion by the Dalai Lama group".
According to Jonathan Watts in the Guardian, "Chinese journalists and student groups have been told to scale back their participation because images of cities and campuses turning dark do not fit the upbeat propaganda message that the authorities wanted to convey.... on the anniversary of the entry into Tibet of the People's Liberation Army and the fleeing of the Dalai Lama into exile."
Get ready to party! Source
Some cities in China are participating; Shanghai is turning out the lights. And some are subverting the rules a bit:
At Beijing University, the authorities have forbidden students from overtly participating in Earth Hour. Instead they will organise a "star-gazing" evening that will require them to turn off the lights to see the heavens more clearly.
More in the Guardian