That the commissioners shall form one board, and a majority of them shall be competent to perform all the duties prescribed, unless otherwise provided. At their first meeting they shall fix stated days or times of meetings, which shall be as often, at least, as once in every two months. Special meetings of the commissioners may also be held on the call of the mayor or a majority of the commissioners. And of every such meeting, when called by the mayor, all the commissioners shall be notified, and when called by a majority of the commissioners, such as shall not join in the call shall be notified.