The holiday is known as Juneteenth, or Juneteenth Independence Day, or also sometimes called Freedom Day. At present, 45 states continue to have observances of Junteenth day annually on June 19.
June 19 was the date on which the emancipation of slaves was announced in Texas in 1865. President Abraham Lincoln's Emancipation Proclamation (September 22, 1862) had set January 1, 1863 as the effective date for freeing all slaves in the Confederate States of America. But the Civil War had to be won to make that promise a reality. Texas was a more isolated territory in those days, and not as much involved in the main Civil War battles. There were, however, slaveholders in Texas and about a quarter-million slaves living there by 1865. General Robert E. Lee of the Confederate states had surrendered in April, 1865, but fighting in the western regions continued till June. On June 18, General Gordon Granger of the Union Army arrived in Galveston, Texas with Union troops, and on announced the emancipation of slaves. That's the background of why June 19th was chosen as the day for celebrating the actual emancipation of all slaves.
In the CivilWar era, this struggle focused heavily on the institution of slavery and whether the federal government had the right to regulate or even abolish slavery within an individual state.
