Richard Nixon actually said...
The provisions of this order shall apply to officers and employees of the Post Office Department and the United States Postal Service (except that sections 3, 4, 5, and 6 shall not apply to the Postal Field Service) until changed by the Postal Service in accordance with the Postal Reorganization Act.
Context
Executive Order 11582—Observance of Holidays by Government Agencies
02/10/1971