Juneteenth commemorates the announcement of federal orders in Galveston, Texas, on June 19, 1865, proclaiming that all people held as slaves in Texas were free. It's been celebrated and recognized as a holiday in various iterations since it's first anniversary.
This Smithsonian article gives a good background and context for this year's Juneteenth: https://www.smithsonianmag.com/history/juneteenth-our-other-independence-day-16340952