The Bird's Nest is the showpiece stadium in Beijing
The Beijing Olympic Stadium will not be ready until mid-April because of delays with the final preparations.
The 91,000-seater Bird's Nest was originally set to be completed at the end of last year before the date was revised for the end of March.
"The finishing touches and requirements needed for the opening and closing ceremonies means the stadium is not ready," said a spokesperson.
"But it will be ready for the first test event on 18 April."
That event will be the IAAF Walking Challenge, which will be followed by an athletics meeting on 22-25 May.