Section 6. Within thirty days of receipt of certification by the board of registrars of voters that a petition contains sufficient valid signatures the city council or board of selectmen shall by order provide for submission of the question as specified in the petition to the voters of the city or town at the first regular city election, or at the first annual or biennial town meeting for the election of town officers, held on or after the sixtieth day following the adoption of the order. Said order shall not require the concurrence of the mayor in a city and shall not be subject to referendum. If an order of the city council or board of selectmen under this section has not been adopted within the thirty days specified above, the question contained in the petition shall be submitted to the voters at the first regular city election, or the first annual or biennial town meeting for the election of town officers, held on or after the ninetieth day after receipt by the city council or board of selectmen of certification provided for in the first sentence of this section.