Mass. Gen. Laws ch. 48 § 57
Subject to section fifty-nine, members of the fire department of every city which accepted chapter five hundred and forty-six of the acts of nineteen hundred and twelve or chapter ninety-seven of the General Acts of nineteen hundred and fifteen, shall be excused from duty for one day out of every five days, without loss of pay. The time and manner of so excusing members shall be determined by the chief, or other officer or board at the head of the fire department, who shall have authority, in case of any public emergency, to prevent any member of the department from taking the day off herein provided for at the time when he is entitled thereto or at the time assigned therefor; but such day off shall be granted to him as soon thereafter as practicable, and such days shall be in addition to any annual vacation allowed to the members of said departments, and such annual vacations shall not be diminished on account of the days off herein provided for.
Mass. Gen. Laws ch. 48, § 57