This is also called Freedom Day in the US because this is the day when slavery was supposedly removed. Slavery used to be extremely rampant in the USA. There were some African Americans who were forcibly taken from their homes in order to become slaves in America. On June 19, 1865, the soldiers made it a point to free the remaining slaves in Texas to the joy and relief of most of the slaves.
The fact that they were freed that day signifies that the USA is a land for the free no matter what your skin color is. The USA right now is full of people from various races that are also considered as Americans. They deserve the same treatment as White Americans too.