Mass. Gen. Laws ch. 43 § 83
The city council shall have and exercise all the legislative powers of the city, except as such powers are reserved by this chapter to the school committee and to the qualified voters of the city.
The city council, elected as aforesaid, shall meet at ten o'clock in the forenoon of the first Monday of January following the regular municipal election, and the members of the city council shall severally make oath, before the city clerk or a justice of the peace, to perform faithfully the duties of their respective offices, except that any member-elect not present shall so make oath at the first regular meeting of the city council thereafter which he attends. For the purposes of organization, the city clerk shall be temporary chairman until the mayor or vice-chairman has qualified. The city council shall, by a majority vote of all the members elected, elect a mayor and a vice-chairman from its own members and the persons elected as such shall likewise make oath to perform faithfully the duties of the respective offices to which they are so elected, and they may so make oath at the same meeting at which they are so elected. The organization of the city council shall take place as aforesaid, notwithstanding the absence, death, refusal to serve or non-election of one or more of the members; provided, that a majority of all the members elected to the city council are present and have qualified. If the office of mayor or vice-chairmen becomes vacant, the city council shall in like manner elect one of its members to fill such office for the unexpired term; provided, that no such vacancy shall be filled so long as there is any vacancy in the council.
If, upon the expiration of fourteen days after the first Monday in January following the regular municipal election, the city council has failed to elect a mayor and a vice-chairman, the member of the city council who received the highest number of votes at the preceding municipal election shall become mayor and the member who received the next highest number of votes at said election shall become vice-chairman, and they shall be so declared by the city clerk. The persons becoming mayor and vice-chairman as aforesaid shall make oath to faithfully perform the duties of the respective offices of mayor and vice-chairman and they shall serve until the end of the term for which they were elected as members of the city council. If the office of the mayor and vice-chairman becomes vacant during said term, the member of the council senior in length of service, or, if more than one has so served, then the member senior both in age and length of service shall perform the duties of the mayor until a new mayor has been elected and has qualified.
Mass. Gen. Laws ch. 43, § 83