From the National Postal Mail Handlers Union:
Postmaster General Megan Brennan has declared Wednesday, December 5, 2018, as a Day of Mourning to commemorate the passing of former President George H. W. Bush. Postal facilities either will be closed or on limited operations, in accordance with the December 3rd USPS Link.
In 1998, we obtained a national arbitration award that granted all mail handlers administrative leave (even if they were not scheduled to work) for the Day of Mourning declared by former President Clinton and then Postmaster General Runyon upon the passing of former President Richard Nixon. Two years later, in May 2000, the parties at the National level agreed to a Memorandum of Understanding, which guaranteed such administrative leave should another National Day of Observance or National Day of Mourning be declared at some point in the future, first by the President of the United States by Executive Order and then by the Postmaster General.
As noted in the USPS Link, the pay and leave administration for the National Day of Mourning for affected bargaining unit employees will be governed by the provisions of the May 4, 2000 Memorandum of Understanding and Section 519.4 of the Employee and Labor Relations Manual. A copy of that MOU is attached for your easy reference.