March 15, 1820

Yes, that would be the Missouri Compromise. I just got the time frame wrong because I simply related the two compromises as direct predecessors of the Civil War. This is from Atomica:

Missouri Compromise, 1820–21, measures passed by the U.S. Congress to end the first crisis concerning the extension of slavery. Maine was admitted as a free state and Missouri as a slave state, and slavery was prohibited from the Louisiana Purchase north of 36[deg]30′. This proviso held until 1854, when the Kansas-Nebraska Act repealed the Missouri Compromise.