Public Resources Code § 5526
ARTICLE 3. Regional Park, Park and Open-Space, and Open-Space Districts
No informality in any proceeding or in the conduct of the election, not substantially affecting adversely the legal rights of any person, shall be held to invalidate the incorporation of any district, and any proceedings in which the validity of the incorporation is attacked shall be commenced within three months from the date of filing the order described in section 5524 with the Secretary of State; otherwise, incorporation and the legal existence of the district is valid and in every respect legal and incontestable.