During the progress of the construction of any improvement under contract, the directors shall maintain a regular office within such district, and may in their discretion when deemed necessary, maintain a regular office in the district during any other time. The directors shall hold an annual meeting on the first day of December at ten o’clock A.M. and may provide for meetings at stated intervals by resolution duly passed, and the president or any two directors may call special meetings at any time that may be deemed proper or necessary.