Martin Luther King Jr. Day is a federal holiday in the United States, observed on the third Monday of January each year to honor the legacy of civil rights leader Martin Luther King Jr. Here are 10 interesting facts about this important holiday:

1. Martin Luther King Jr. Day was first observed as a national holiday in 1986, after legislation was signed into law by President Ronald Reagan. However, it wasn't until 2000 that all 50 states officially recognized the holiday.
2. The holiday was established to honor the birthday of Martin Luther King Jr., who was born on January 15th, 1929.
3. Martin Luther King Jr. Day is one of only four federal holidays in the United States that are officially designated as "national holidays" rather than "legal holidays."
4. The holiday is also observed as a state holiday in some states, such as California and New York, and as a civic holiday in some cities and municipalities.
5. The holiday is marked by a variety of events and activities, including parades, community service projects, and educational programs. Many organizations and schools also use the day to highlight King's contributions to the civil rights movement and his message of nonviolence and social justice.
6. The holiday is also a day of service, where many people participate in community service projects and volunteer work. This is a way of honoring King's message of social justice and civil rights.
7. Martin Luther King Jr. Day is also observed as a day of remembrance and reflection, with many churches and religious organizations holding special services and ceremonies to honor King's legacy.
8. The holiday was celebrated for the first time on January 20, 1986, three years after King's death.
9. It was the first national holiday to honor an African American.
10. Despite being a federal holiday, not all states observe MLK Day as a paid holiday for state employees. Some states have chosen to combine it with other holidays, such as Robert E. Lee's birthday, or to replace it with other holidays honoring Civil War figures.

Martin Luther King Jr. Day is an important reminder of the struggles faced during the Civil Rights Movement, and the legacy of one of the most important figures in American history.