Lincoln issued a preliminary proclamation on September 22, 1862, after the Battle of Antietam. It warned Confederate states that enslaved people would be declared free if they did not rejoin the Union by the new year. None did. The final order took effect on January 1, 1863, as a war measure.
The proclamation had limits. It exempted the slaveholding border states of Delaware, Kentucky, Maryland and Missouri, and parts of the Confederacy already under Union control. Freedom depended on the Union Army arriving, which is why enslaved people in Texas did not learn of it until June 19, 1865. About 180,000 Black men served in the Union Army, which turned the war into a fight against slavery.
Because it was a wartime order, it could have been challenged after the war. The Thirteenth Amendment, ratified in December 1865, made the end of slavery permanent and nationwide.