On days when the federal government isn't doing business, neither is the post office. The U.S. Postal Service follows the same holiday schedule as federal offices.
In 2021, mail won't be delivered on Sundays, nor the following dates: Jan. 1, Jan. 18, Feb. 15, May 31, July 5, Sept. 6, Oct. 11, Nov. 11, Nov. 25, and Dec. 25. (July 5, the day after Independence Day, will be treated as a holiday by the USPS.)
Please keep in mind that occasionally, particularly near holidays, your local post office may be open, but closes earlier in the day than usual.
Here's a complete list of federal holidays and the corresponding dates in 2021 when post offices will be closed:
Download the entire list of post office holidays as a PDF