Mass. Gen. Laws ch. 43 § 11
If a majority of the total number of votes cast at a regular city election for and against the adoption of one of the plans of government provided for in this chapter shall be in favor of its adoption, this chapter, so far as applicable to the form of government under the plan adopted by the city, shall supersede the provisions of its charter and of the general and special laws relating thereto and inconsistent herewith, but not, however, until officers provided for under such plan shall have been duly elected and their terms of office shall have begun. The officers provided for under the plan so adopted shall be elected in accordance with the provisions of this chapter relating to such plan and in accordance with section fifteen, and their terms of office shall begin at ten o'clock in the forenoon of the first Monday of January following their election.
Mass. Gen. Laws ch. 43, § 11