Juneteenth is the oldest known celebration of the ending of slavery. Dating back to 1865, it was on June 19th that the Union soldiers, led by Major General Gordon Granger, landed at Galveston, Texas with news that the war had ended and that all slaves were now free. Note that this was two and a half years after President Lincoln's Emancipation Proclamation - which had become official January 1, 1863.
Not surprisingly, many slaves throughout the Southern states had been kept ignorant about the Emancipation Proclamation. General Granger's announcement in Galveston capped off a 2 1/2 year march throughout the South, and signified the real end of slavery.
On the night of June 19th, thousands of freed slaves in Galveston celebrated with music, dance, and food. While Juneteenth began as a regional celebration, today it's celebrated across the country.