On the day of the election the polls at each of the polling places designated by the board of supervisors shall be opened at the hour of seven o’clock a.m. and shall be kept opened until the hour of eight o’clock p.m. of the same day, when the polls shall be closed. Any elector within the polling place or standing in line thereat who has not had an opportunity to vote and desires to vote shall be permitted to vote after the hour of eight o’clock p.m. of the day of election.