Today is Christmas Eve. Among the agencies affected: Mail: Most post offices will be open until noon and mail will be delivered. Banks: Most banks and savings and loan institutions will be open. Courts: All courts will be closed. Schools: Most public schools in Los Angeles County--including those in the Los Angeles Unified School District--will be closed until Jan. 4. Government: All federal, state, county and municipal offices--except those providing emergency services--will be closed.