Mass. Gen. Laws ch. 43, § 88: Vacancies

Section 88. Except as provided in this section, a vacancy in any elective body shall be filled for the unexpired term by a majority vote of the remaining members, except that if the remaining members fail to fill such vacancy within thirty days after they shall have been notified by the city clerk that such vacancy exists, such vacancy shall be filled by the appointment of any qualified voter of the city by the mayor, or, if there is no mayor, by the vice-chairman, or the vice-chairman of the school committee, as the case may be.