Juneteenth is a holiday commemorating the abolition of slavery in the United States. The Emancipation Proclamation was issued by Abraham Lincoln on September 22, 1862 and went into effect on January 1, 1863.
Sooooo, why celebrate in June?
Well, it’s a Texas thing.
On June 18, 1865 the Union Army arrived in Galveston, Texas. The next day, June 19, General Gordon Granger read General Order Number 3 which stated, "The people of Texas are informed that in accordance with a Proclamation from the Executive of the United States, all slaves are free. This involves an absolute equality of rights and rights of property between former masters and slaves, and the connection heretofore existing between them becomes that between employer and free laborer."
Yes, news of emancipation moved slowly. General Order Number 3 didn't reach Texas until two years after President Lincoln issued the Emancipation Proclamation. General Lee and the Army of Northern Virginia surrendered to General Grant at Appomattox in April 1865, which means General Granger had to deal with ongoing slavery after the war ended.
But time and the size of Texas could not stop emancipation from rolling across the land. The first public Juneteenth events occurred in 1866. At these events, former slaves read the 1863 Emancipation Proclamation, sang spirituals, held games, and celebrated freedom. Juneteenth celebrations still take place all over Texas today.
Juneteenth has been an official state holiday in Texas since 1980. You can read more about Juneteenth from the Handbook of Texas.