When a vacancy shall take place in the office of the mayor a successor shall be elected for the unexpired term at the next municipal election occurring more than thirty days after the commencement of such vacancy, unless such election shall occur in the last year of said term, in which case a mayor, pro tempore, shall be chosen by council, in convention, by votes of the majority of members elected, and the person elected mayor shall hold office until the expiration of said term, and until his successor shall be duly elected and qualified, and it shall be the duty of the president of the council to issue a proclamation for such joint convention, to be held not less than ten days nor more than twenty days after such vacancy shall take place.