From History.com:

Juneteenth is the oldest nationally celebrated commemoration of the ending of slavery in the United States. Dating back to 1865, it was on June 19 that the Union soldiers, led by Major General Gordon Granger, landed at Galveston, TX, with news that the war had ended and that the enslaved 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. The Emancipation Proclamation had little impact on the Texans due to the minimal number of Union troops to enforce the new Executive Order. However, with the surrender of General Lee in April of 1865, and the arrival of General Granger’s regiment, the forces were finally strong enough to influence and overcome the resistance.