| Juneteenth is the oldest known celebration commemorating the ending of slavery in the United States. On June 19, 1865, the end of slavery and the Civil War were finally announced in the westernmost reaches of the country. Read by Major General Gordon Granger, General Order No. 3 was delivered in Galveston, TX, effectively ending American slavery.
Thus began a tradition of jubilant celebration that is now recognized or observed in nearly every state and the District of Columbia.
Learn more at Juneteenth.com, the National Juneteenth Observance Foundation, in this article from Smithsonian Magazine, and in this Fact Sheet from the Congressional Research Service.
*UPDATE* On June 17, 2021, President Biden signed legislation making Juneteenth a federal holiday to commemorate the end of slavery in the United States.