Written protests. In case of a written protest against any proposed amendment of the regulations or districts, signed and acknowledged by either: (1) the owners of 20% of the frontage proposed to be altered; (2) the owners of 20% of the frontage immediately adjoining or across the alley therefrom; or (3) the owners of 20% of the frontage directly opposite the frontage proposed to be altered, is filed with the Village Clerk on or before the date of the public hearing set forth in the notice of hearing so published, the amendment shall not be passed except by an affirmative vote of 2/3 of the Village Board then holding office. In such cases, a copy of the written protest shall be served by the protestor(s) on the petitioner for the proposed amendment and a copy upon the attorney, if any, for the petitioner, by certified mail at the address of such petitioner and attorney as shown in the petition for the proposed amendment.