"Juneteenth commemorates June 18 and 19, 1865. June 18 is the day Union General Gordon Granger and 2,000 federal troops arrived in Galveston, Texas, to take possession of the state and enforce the emancipation of its slaves " (Wikipedia). June 19, 1865 is when the Emancipation Proclamation was read by Gen. Granger.
Until then, many slaves did not know they were legally free. As of June 2012, 41 states and the District of Columbia have recognized Juneteenth as either a state holiday or state holiday observance (see list of states at Wikipedia under Juneteenth).
Juneteenth is an interesting and important part of American history and should be observed by all Americans. Not only were slaves freed by the Proclamation, so were all citizens of the United States of America.