Question

why do we celebrate martin luther king, jr. day?
he ended slavery.
he was a nice man.
he played an important role in the civil rights movement.
he was a president.

Explanation:

Martin Luther King, Jr. was a central leader of the U.S. Civil Rights Movement, advocating for racial equality through nonviolent means. Ending slavery was achieved by Abraham Lincoln, King was not a president, and "being a nice man" is not a formal, historically significant reason for the national holiday.

Answer:

He played an important role in the Civil Rights Movement.