As soon as may be after the twelfth day after notice that a petition has been directed to be heard by the board has been served upon the adverse parties thereto, the board shall fix a time and place for hearing the petition. If a petition be assigned to a referee, he shall, as soon as practicable thereafter, fix a time and a place for hearing the petition. The secretary of the board, if the petition has been directed to be heard by the board or by one or more of its members, or the referee to whom the petition has been assigned, shall serve upon all parties in interest a notice of the time and place of hearing, and shall serve upon the petitioner a copy of any answer of any adverse party.