This is an African American holiday started in Texas, for obvious reasons. Wikipedia has a nice summary of key info:
Juneteenth, also known as Freedom Day or Emancipation Day, commemorates the announcement of the abolition of slavery in the U.S. State of Texas in 1865. Celebrated on June 19, the term is a portmanteau of June and nineteenth, and is recognized as a state holiday in 31 of the United States.
That is, word of President Lincoln’s emancipation proclamation of January 1863 reached Texas only in June 1865:
The holiday originated in Galveston, Texas; for more than a century, the state of Texas was the primary home of Juneteenth celebrations. Since 1980, Juneteenth has been an official state holiday in Texas. It is considered a “partial staffing holiday” meaning that state offices do not close but some employees will be using a floating holiday to take the day off. Twelve other states list it as an official holiday, including Arkansas, New York, Massachusetts, Connecticut, Alaska and California, where Governor Schwarzenegger proclaimed the day “Juneteenth” on June 19, 2005. Connecticut, however, does not consider it a legal holiday or close government offices in observance of the occasion. Its informal observance has spread to some other states, with a few celebrations even taking place in other countries.
As of May 2009, 31 states and the District of Columbia have recognized Juneteenth as either a state holiday or state holiday observance; these include Alaska, Arkansas, California, Colorado, Connecticut, Delaware, Florida, Idaho, Illinois, Iowa, Kansas, Louisiana, Massachusetts, Michigan, Missouri, New Jersey, New Mexico, New York, North Carolina, Oklahoma, Oregon, Tennessee, Texas, Vermont, Virginia, Washington and Wyoming.
This is also a day to remember the 500,000 African Americans, who as soldiers and support troops, many of them formerly enslaved, volunteered for the Union Army at its low point, and who thus played a (the?) key role in winning the Civil War. This is an ironic day, too, given the very weak apology for slavery voted on this week in the mostly white US Senate. A bit late.