Mass. Gen. Laws ch. 147 § 16C
Members of the police department of every city or town which accepts this section by vote of the city council, subject to the provisions of the charter, or by vote of the town at an annual town meeting, as the case may be, or, if said city council or town fails to accept this section, by vote of the voters thereof as hereinafter provided, shall be excused from duty for two days out of every seven without loss of pay.
Upon petition of not less than ten per cent of the registered voters in a city or town, duly certified by the registrars of voters and filed with the city or town clerk not less than thirty days before the regular municipal election of such city in any year, or before the annual town election of such town in any year, or if such petition is filed with the state secretary not less than ninety days before a biennial state election, the city or town clerk or the state secretary, as the case may be, shall cause to be printed upon the official ballot to be used in such city or town at such election the following question:
"Shall the city (or town) accept the provisions of section sixteen C of chapter one hundred and forty-seven of the General Laws providing for a five day week for members of the police forces in cities and towns?". [YES.][ ] [ NO.][ ]
If a majority of the votes in answer to said question in any city or town is in the affirmative, this section shall take full effect therein, but not otherwise.
Mass. Gen. Laws ch. 147, § 16C