The board of commissioners shall hold regular meetings on the first Monday of December, January, February, March, June and September in each year, except that when such a regularly scheduled meeting would fall on a legal holiday under Section 17 of “An Act to revise the law in relation to promissory notes, bonds, due bills and other instruments in writing”, approved March 18, 1874, as amended, the board shall hold the meeting on the day immediately following such a holiday. It shall be the duty of the president of the board of commissioners to call special meetings of the board whenever, in his opinion, the same may be necessary; and he shall preside at all the meetings of the board, and generally perform the duties usually performed by a presiding officer; provided, that in the absence of the president, or of his inability to act, a president pro tempore may be elected, who shall, during such absence or inability possess all the powers and perform all the duties by law vested in and required of the president.