California Code, Government Code - GOV § 54974

(a) Whenever an unscheduled vacancy occurs in any board, commission, or committee for which the legislative body has the appointing power, whether due to resignation, death, termination, or other causes, a special vacancy notice shall be posted in the office of the clerk of the local agency, on either the local agency's Internet Web site or at the library designated pursuant to Section 54973, and in other places as directed by the legislative body, not earlier than 20 days before or not later than 20 days after the vacancy occurs.  Final appointment to the board, commission, or committee shall not be made by the legislative body for at least 10 working days after the posting of the notice in the clerk's office.

(b) Notwithstanding subdivision (a), the legislative body may, if it finds that an emergency exists, fill the unscheduled vacancy immediately.  A person appointed to fill the vacancy shall serve only on an acting basis until the final appointment is made pursuant to this section.