Mass. Gen. Laws ch. 41 § 107
A person who is elected town clerk shall be sworn either by the moderator or by a justice of the peace, and shall enter upon the performance of his duties on the seventh day succeeding his election or as soon thereafter as he is qualified and shall hold office during the term fixed by law, which shall begin on the seventh day succeeding his election, and until another person is qualified in his stead. Every person elected as a moderator of a town or district or to any other town office designated by name in section one, or as an officer of a district, which district includes two or more towns, before entering upon his official duties shall be sworn to the faithful performance thereof either by the town clerk in the case of a moderator, and in all other cases by the moderator, or by the town clerk in the town in which he resides, and, unless other provision is specifically made by law, shall enter upon the performance of his duties on the day after his election, or as soon thereafter as he is qualified, and shall hold office during the term fixed by law, which shall begin on the day after said election, and until another person is qualified in his stead. Every other elected member and every appointed member of every board or commission of a town, and every other elected officer and every appointed officer of a town, shall also, before entering upon his official duties, be sworn to the faithful performance thereof.
Mass. Gen. Laws ch. 41, § 107