Originally, the date (June 19) celebrated the 1865 arrival in Galveston of news of the Emancipation Proclamation, President Lincoln’s then more than 2-year-old executive order freeing the slaves of the Confederacy.
Gradually the date came to be celebrated elsewhere as well and take on a broader meaning. In Texas, it became a state holiday in 1980. In Austin, it has long been commemorated by a (generally) peaceful march/parade of some kind. Hope it’s peaceful this year. It likely won’t be elsewhere.
UPDATE: An alternative view of “the high holy day of anti-white hatred.”
MORE: A San Francisco Juneteenth mob toppled statues of Francis Scott Key, author of the anthem those NFL players kneel to, and Ulysses S. Grant. Grant? Savior of the Union and Emancipation? Brilliant.
THE TEXAS SCRIBBLER 
Yes, sure, some statues are falling, but in the mean time the city of Gelsenkirchen has erected a new one. Take it easy, go with the flow, goodbye racist Churchill, hello noble Lenin…
https://www.welt.de/politik/deutschland/article209975317/MLPD-enthuellt-Lenin-Statue-in-Gelsenkirchen.html
Hitler would be aghast.