Every Saturday from noon to midnight is a holiday as regards the transaction of business in the public offices of the state and political divisions where laws, ordinances, or charters provide that public offices shall be closed on holidays. This section shall not be construed to prevent or invalidate the issuance, filing, service, execution, or recording of any legal process or written instrument during such period. Public offices of a city shall be closed on those holidays enumerated in Section 6700 unless otherwise provided by charter, ordinance or resolution.