law: 15. Filling of vacancies.
  (1) Vacancies arising by efflux of time in the office of an elected member of a Board shall be filled by an ordinary election to be held on such date as the Central Government may, by notification in the Official Gazette, direct.
  (2) A casual vacancy shall be filled by a casual election the date of which shall be fixed by the Central Government by notification in the Official Gazette, and shall be, as soon as may be, after the occurrence of the vacancy:
  Provided that no casual election shall be held to fill a vacancy occurring within six months of any date on which the vacancy will occur by efflux of time, but such vacancy shall be filled at the next ordinary election.