Mass. Gen. Laws ch. 39 § 5
Except as otherwise provided by city charters, upon the death, resignation or absence of the mayor, or his inability to perform the duties of his office, the president of the board of aldermen shall perform them; and if there is no such officer, or if he also is absent or unable from any cause to perform them, they shall be performed by the president of the common council, or, if there is no such officer, or if he is absent or unable to perform such duties, by such alderman as the board of aldermen may from time to time elect, until the mayor or the president of the board of aldermen is able to attend to said duties or until the vacancy is filled. The person upon whom such duties devolve shall be called "acting mayor" and shall possess the powers of mayor only in matters not admitting of delay, and shall not make permanent appointments.
Mass. Gen. Laws ch. 39, § 5