According to a letter sent by General Assembly President Peter Thomson on Tuesday, the date for the appointment has been set for October 13 at UN headquarters, Xinhua news agency reported.
The term of office for Guterres will start on January 1 next year and end on December 31, 2021, said the letter.
Thomson will also convene an informal plenary meeting of the 193-member General Assembly with the secretary-general-designate on October 19, according to the letter.
On Thursday, the UN Security Council formally and unanimously recommended to the General Assembly Guterres, former Portuguese prime minister and former head of the UN refugee agency, be appointed the next secretary-general.
Under the UN Charter, the Secretary-General shall be appointed by the General Assembly upon the recommendation of the Security Council.
In practice, the Security Council, particularly its five permanent members, will make the final choice and send a single candidate to the General Assembly for approval.
Guterres, a 67-year-old politician, served as UN High Commissioner for Refugees from June 2005 to December last year.
Before joining the UN refugee agency, Guterres spent more than 20 years in government and public service. He was the prime minister of Portugal from 1995 to 2002.