Juneteenth originates from Galveston, Texas when in 1865 African Americans who had been enslaved in Texas were finally emancipated. President Abraham Lincoln's Emancipation Proclamation in 1862 outlawed slavery in the Confederate States, but it took 3 more years for Galveston, Texas to recognize it officially on June 19th, 1865.
Still, it ought to be recognized that the Emancipation Proclamation only emancipated African Americans in the Confederate States, not the Union States. Slavery was still legal and practiced in two Union border states, Delaware and Kentucky, until December 6, 1865, when ratification of the 13th Amendment to the Constitution abolished chattel slavery nationwide. Additionally, Native American territories that had sided with the Confederacy were the last to release those enslaved, in 1866.
Below you'll find resources that explain the entire history behind Juneteenth and ways to celebrate and be an ally:
The Historical Legacy of Juneteenth
How to Say Happy Juneteenth
Kids' Books to Celebrate Juneteenth
A History, Celebrations, and Resources