Martin Luther King Jr. Day, a federal holiday designated as a national day of service to encourage all Americans to volunteer and improve their communities, according to the National Museum of African American History and Culture.

Martin Luther King Jr. Day is a federal holiday, but some shipping and delivery services may still be operational. Here's what we know.

Here is everything you need to know about the post office, mail delivery, FedEx, UPS and Amazon on the holiday. Those who are hoping to stop at the post office that day will have to wait until Tuesday, Jan. 21, to visit. People who want to send out mail on MLK Day will also have to wait until Tuesday.