Once the civil war was ended and slavery was officially outlawed, how was that law implemented and enforced? Were there areas of the country that started off unwilling to enforce the law in which federal troops had to be stationed to enforce abolition? Were there years or even decades that plantation owners were caught still keeping slaves in secret? Did many former slaves stay with their former masters as low-paid free workers? Was there a transition period for plantation owners to adjust to the new laws or overnight was it just declared, "all slaves are officially free, tomorrow you have to let them walk off of your property and go wherever they want"? Once a former slave was freed, where did they go and what did they do? Day one of freedom, how did they immediately come up with food and shelter?
The Thirteenth Amendment, which officially abolished slavery throughout the US, was ratified on 6 December 1865. On that day, slavery was immediately abolished without any transition period. However, the amendment had passed Congress the previous January, and I think it would have been obvious it would be ratified.
However, this only affected Kentucky and Delaware. In the rest of the South, slavery had already been abolished - either by the Reconstruction governments (in Tennessee and parts of Louisiana, among other areas) or by the Emancipation Proclamation. President Lincoln issued a preliminary proclamation on 22 September 1862, warning the South that unless they ended their rebellion by 1 January, he would free their slaves; on 1 January, he did so for all regions not yet reconquered.
Of course, emancipation wasn't enforced in any given region until the US Army reconquered it; when they did, though, all slaves were immediately set free. Frequently, they would follow the army, either as camp followers or enlisting as soldiers. Such large-scale migration continued through 1865, as freedmen searched for better conditions or for family members who'd been sold elsewhere before the war. Many others, though, did stay with their former masters, at first as wage laborers doing the exact same work as before emancipation. Many overseers even tried to continue whipping freedmen. A combination of protests to the Freedmen's Bureau and mass dissatisfaction first ended whipping, and then gradually caused planters to end gang labor in favor of sharecropping - which the freedmen preferred because each family was able to care for an individual field.
The US Army and the members of the Freedmen Bureau had to come down South in order to make sure the 13th amendment and other civil rights legislation was enforced.
Otherwise, the southerners would be still free to abuse freed slaves and prevent them from excising their right as free citizens.