(NEXSTAR) – As the calendar turns to the third Monday of January, countless Americans will pause to honor Martin Luther King Jr. Day.

In 1983, President Ronald Reagan signed a landmark bill into law, establishing this federal holiday. This year, we commemorate 40 years since the inaugural observance of Martin Luther King Jr. Day, as highlighted by the National Museum of African American History & Culture.

Although not all businesses recognize MLK Day with a day off, federal agencies do take part, which means the U.S. Postal Service will halt regular mail deliveries on Monday. Post office branches will remain closed, although Priority Mail Express services will continue to operate as usual.