The Constitutional Convention was an event mandated by the Corellian Treaty. According to the Treaty, the Convention—deciding the form of the Second Galactic Republic—was mandated to occur once Palpatine resigned, was deposed, or died.
The Convention was convened by the Alliance High Command following the Battle of Endor to debate the creation of the New Republic. It lasted for months with all delegates getting a chance to speak, but nothing was decided. In the end the convention established a provisional government under the New Republic Provisional Council to establish a permanent constitution and government.