In the middle of the American Civil War in 1863, President Lincoln signed the Emancipation Proclamation, announcing that all slaves in the United States were freed (with the exception of four Pro-Union border states – Missouri, Kentucky, Maryland, and Delaware.) It took two more years for the 13th amendment to be passed, which prohibited slavery and unpaid labor (again, with notable exceptions that exist to this day, namely, the prison population.)
Juneteenth commemorates June 19th, 1865, when Union troops marched into Galveston Bay, Texas, and informed the roughly quarter-million enslaved locals that they were, in fact, free.