On Thursday, his lawyers Chen Guangwu and Zhang Xuezhong said they would not attend the court proceedings because they could not mount an effective defense after authorities did not let them copy court files crucial to the case, including videos and photos related to Yang's alleged offenses.
The court did not provide a reason for the refusal, Chen said.
Through a statement released by his lawyers late Thursday, Yang said the court had unlawfully deprived him and his legal team of a proper defense. He vowed to remain silent if the trial went ahead.
Calls to the court after the postponement rang unanswered.
New York-based Human Rights Watch urged Chinese authorities on Friday to release both Yang and Sun.
"Much of Guo's and Sun's work echoed authorities' stated policy goals, such as fighting corruption, so where's the evidence they violated any Chinese law?" Sophie Richardson, the group's China director, said in a statement.
In January 2013, Yang helped organize demonstrations and gave a speech in support of the editorial staff at the newspaper Southern Weekly in Guangzhou after the journalists said a New Year's message that called for rule by the constitution was altered because of censorship.
Authorities have accused Yang of disturbing public order, but his supporters say the rallies were orderly.
Yang's lawyers said he denies any guilt.
