Why do we celebrate Martin Luther King Jr Day as a national holiday?

Related questions

1. Is Target closed on MLK Day?
2. Did UPS deliver today?
3. Can you track the UPS truck?
4. Are businesses closed on MLK Day?
5. Does USPS run on MLK Day?
6. Is Target closed on Christmas?
7. Who gets MLK day off?
8. Is there mail service on Martin Luther King Day?
9. Is Martin Luther King Day a national holiday?
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2. #martin
3. #united
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5. #equality
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13. #luther
14. #leader
15. #segregation
16. #rights
17. #civil
18. #american