Hello there!
Your answer would be September 20, 1850.
On that day, Congress signed to abolish the slavery trade that was in the District of Columbia in a legislative package called the Compromise of 1850. Even though the slave trade has discontinued, the ownership of slaves continued since this law did not apply to slave ownership. Especially people that live in southern parts of the United States were not happy for things like this to happen, and they wouldn't like slavery to be abolished for good.
