From the US Postal Service:
The U.S. Postal Service is fully supportive of the new Juneteenth National Independence Day Act and making June 19 a federal holiday. Unfortunately, it is not possible to cease the operations of the Postal Service to accommodate an observance over the next 24-48 hours.
We are part of the nation’s critical infrastructure and our customers are relying on us to deliver our essential services. Closing down our operations without providing appropriate time would lead to operational disruptions and be a disservice to our customers and those who rely upon us. For that reason, the Postal Service will be operating on June 18 and 19, 2021, on a normal schedule, serving our customers to the best of our ability.
The Postal Service will discuss future recognition of this significant new holiday with our national Unions, Management Associations, and other stakeholders, consistent with our collective bargaining and consultative obligations.
Source: USPS Operating Status for June 18 and 19, 2021 – Newsroom – About.usps.com