Mass. Gen. Laws ch. 43, § 17: Oath of office; time of taking

Section 17. On the first Monday in January following a regular municipal election, at ten o'clock in the forenoon, the mayor-elect if elected by the people, the councillors-elect, and the assessors-elect if elected by the people, shall meet and be sworn to the faithful discharge of their duties. The oath may be administered by the city clerk or by a justice of the peace, and a certificate thereof shall be entered in the journal of the city council. At any regular council meeting thereafter the oath may be administered in the presence of the city council to the mayor, or to any councillor absent from the meeting on the first Monday in January; provided, that, under Plan E, the oath may be so administered to the mayor and vice-chairman at the same meeting at which they are respectively elected.