(a) If a vacancy in office is to be filled by special election, the election shall be ordered as soon as practicable after the vacancy occurs, subject to Subsection (b).
(b) For a vacancy to be filled by a special election to be held on the date of the general election for state and county officers, the election shall be ordered not later than the 78th day before election day.
Official text of the Texas Election Code as published by the Texas Legislative Council.