I'm not sure that many Americans knew what it was either. Juneteenth is the date commemorating Union army general Gordon Granger announcing federal orders in Galveston, Texas, on June 19, 1865, proclaiming that all people held as slaves in Texas were free.[6] Although the Emancipation Proclamation had officially freed them almost two and a half years earlier, and the American Civil War had largely ended with the defeat of the Confederate States in April, Texas was the most remote of the slave states, with a low presence of Union troops, so enforcement of the proclamation had been slow and inconsistent.[6]
https://en.m.wikipedia.org/wiki/Juneteenth
That the Trump administration chose this date for their rally in itself isn't the Dog Whistle, it is where the rally is being held coinciding with that date. Tulsa.
Tulsa is the location of the worst known racial violence in America. The Tulsa race massacre of 1921, a two-day explosion of violence in which white Oklahomans killed hundreds of African-Americans and burned businesses in a prosperous district known as "Black Wall Street." The bloody attack is considered "the single worst incident of racial violence in American history," according to the Oklahoma Historical Society.
https://www.nbcnews.com/pop-culture...e-1921-exposing-viewers-ugly-chapter-n1069426