Federal Holiday

Slavery ended legally in the confederate states on September 22, 1862 when Lincoln’s Emancipation Proclamation freed all slaves in the rebelling states effective January 1^st, 1863. Of course in practice the proclamation took effect in confederate territories only as the Union army occupied them and enforced it. Slavery was banned everywhere (there were a couple of hold out, non-confederate border states that had  not banned slavery on their own) when the 13^th Amendment was ratified by the last necessary state legislature on December 6th, 1865 after passing congress in January of that year. Either of these significant dates would have been a natural choice for a federal holiday commemorating the end of slavery.

After the Civil War ended in 1865, it took a while for units of the Union army to get to and control all areas of the confederacy.  By June 19^th there was a Union general in Galveston to command the military district of Texas, and on that date he issues a general order informing people in the state that that all former slaves in Texas were free, having been freed by the Emancipation Proclamation. The order was not an insignificant thing. Any former slaves in Texas who did not already know it needed to learn they were free, and the former slave owners surely needed to be told who was running things. But it was only a local order restricted to Texas and announcing an emancipation that in law had already happened.

Over time June 19^th  came to be celebrated as a festive holiday related to the end of slavery, and it is the one congress picked as a federal holiday, which is okay, though one of the other two would have made more sense in terms of historical significance. 

I do agree with the opinions I have heard – sometimes said seriously, sometimes jokingly – that new federal holidays are a good thing in themselves. Any day the employees of the federal bureaucracy are off is a day they cannot be harming and restricting the productive members and useful activities of society.