The first interest payment on the unpaid assessment shall be payable to the treasurer on the April 15th next succeeding the March 1st or the October 15th next succeeding the September 1st, as the case may be, next following the date of the bonds. The following interest payments shall each be for six months interest and shall be payable to the treasurer on each fifteenth day of April and October, the last interest payment coming due the October 15th immediately preceding the last annual payment of the principal of the bonds issued to represent the unpaid assessment.