Whenever a referendum petition bearing signatures equal in number to at least twenty percent of the qualified voters of a municipality, which petition requests that a special election be called to submit the referendum measure to a vote of the people, has been filed with the city clerk and verified pursuant to section 18-2518, it shall be the duty of the governing body of the municipality to reconsider the measure or portion of such measure which is the object of the referendum. If the governing body fails to repeal or amend the measure or portion thereof, in the manner proposed by the referendum, including an override of any veto, if necessary, the city clerk shall cause the measure to be submitted to a vote of the people at a special election called for such purpose within thirty days from the date the governing body received notification pursuant to section 18-2518. Subject to the provisions of section 18-2521, the date of such special election shall be set during the first available month that complies with sections 32-405 and 32-559.