Section 111K. In any city or town which accepts this section, any vacation to which a member of its regular or permanent fire force is entitled shall be granted at such times during the period between the first Sunday in June and the Sunday next following Labor Day as, in the opinion of the board or officer in charge of such force, will cause the least interference with the performance of the regular work of such force; provided, however, that such vacations may be granted at such other periods of the year as shall be mutually agreeable to the member and the board or officer in charge of such force.