Cal. Water Code § 72027

Any action or proceeding in which the validity of the formation of the improvement district or of any of the proceedings in relation thereto is contested, questioned, or denied shall be commenced within three months from the effective date of the resolution of formation, or, if an election is held, within three months from the date of such election; otherwise the formation of the improvement district and all proceedings in relation thereto shall be held to be valid and in every respect legal and incontestable.