Section 57C. Members of the fire department in any city or town accepting the provisions of this section may be excused from duty for not more than two days in any year without loss of pay while in attendance as an official delegate to the convention of the Massachusetts Permanent Firemen’s Association or the Massachusetts State Firemen’s Association held within the commonwealth; provided, however, that any such member who attends both such conventions as an official delegate may be excused from duty for not more than four days in any year.