I don't know where they got that date from. Slavery didn't end in the United States until 6 Dec 1865 with the ratification of the 13th amendment.
"Legally, the last 40,000–45,000 slaves were freed in the last two slave states of
Kentucky and
Delaware[299] by the final ratification of the
Thirteenth Amendment to the Constitution on December 18, 1865. Slaves still held in Tennessee, Kentucky, Kansas, New Jersey,
Delaware,
West Virginia, Maryland, Missouri,
Washington, D.C., and twelve parishes of Louisiana
[300] also became legally free on this date."
https://en.wikipedia.org/wiki/Slavery_in_the_United_States#End_of_slavery
So one could say the end of slavery was on either 6 Dec or 18 Dec 1865 when the last slaves were freed. It certainly wasn't 19 June. This makes me wonder if anyone in congress bothered to check the dates. On 19 June 1865 you still had between 40-45,000 legally owned slaves in different states.
Seems to me if we're going to make the official end of slavery in the U.S. a national holiday, at least they could get the date right. This is kind of like celebrating the 4th of July in January.