The state of South Carolina marked Monday's anniversary by unveiling a new historical marker in downtown Charleston at the site where delegates signed the original Ordinance of Secession.
"For many southerners, the Confederacy is not richly understood, but its seen as a movement resisting outside forces. These people are passionate about their heritage," said Mark Tompkins, a political historian at the University of South Carolina. "Still, it's a murky line to commemorate that heritage while condemning slavery."
Earlier this spring, Mississippi Gov. Haley Barbour and Virginia Gov. Bob McDonnell both sparked public outcry after declaring April "Confederate History Month" but failing to make any mention of slavery in the official proclamations.
McDonnell later apologized for what he called a "major omission" and revised the proclamation to include acknowledgment that "slavery led to this war and was an evil and inhumane practice that deprived people of their God-given inalienable rights."
Critics of the secession gala believe a similar qualification for the celebration should be made.
"Was slavery an issue? Of course it was," said Simpson. "We don't deny that. Slavery was a terrible abomination. But this moment 150 years ago was a remarkable period in history and Americans should study and celebrate it."
The Associated Press contributed to this report.