Mahamat Idriss Deby Itno, who led a military takeover in the Central African nation after his father was killed battling rebels last year, was expected in the Qatari capital by Saturday, diplomats said.
"The signature will be on Monday," said a senior diplomatic source, speaking on condition of anonymity as no official announcement had been made by the Qatari government.
Foreign ministers from countries concerned with stability in Central and West Africa have been invited to the signing, diplomats said.
The main armed opposition Front for Change and Concord in Chad (FACT) has not yet announced whether it will sign the accord.
FACT fighters were among rebels battling the country's longtime president, also named Idriss Deby, when he was killed in April last year.
"Even without FACT, there is a majority in favor of starting the talks in N'Djamena on August 20," said the negotiator for one group.
Qatar has been mediating between dozens of opposition groups and government representatives in Doha since March.
Under the accord expected to be signed on Monday, the military council and rebel groups are to agree a ceasefire while authorities will guarantee security for rebel leaders who attend the national dialogue.
According to a decree signed Thursday by Prime Minister Albert Pahimi Padacke, more than 1,300 representatives of rebel groups, civil society, trade unions and government officials will attend the talks in the Chadian capital.
The start of the national dialogue has been delayed several times since February amid frayed negotiations in Doha.
Chad is seen as a key to Western-led efforts to confront jihadist insurgents in several Central and West African countries.