The date was announced by a Guardians Council spokesman, quoted by the ISNA news agency.
After the previous parliamentary elections in March 2012, reformists virtually disappeared from the assembly -- which now has 209 members -- following a boycott.
The boycott was in response to the repression of protests that erupted following the disputed re-election of Mahmoud Ahmadinejad as president in June 2009, when two major reformist parties were banned.
The reformists returned to politics after the election of President Hassan Rouhani, who is regarded as a moderate, in June 2013.