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.
For 2012, the official days off for postal workers (and, therefore, days that mail won't be delivered) are: Jan. 2, Jan. 16, Feb. 20, May 28, July 4, Sept. 3, Oct. 8, Nov. 12, Nov. 22, Dec. 25.
It's not always obvious that a day will be a postal holiday in any given year. For example, if the legal public holiday falls on a Sunday, such as New Year's Day and Veterans Day do in 2012, the post office will be closed the following day as well. Most of this year's federal holidays fall on a Monday, with the exceptions being Independence Day, Thanksgiving, and Christmas. Here's a complete list:
Are USPS post offices open today? This little web site will tell you whether the post office is closed or open today and tomorrow.