Juneteenth (June 19th) never really made much sense to me, although my hometown in Missouri has celebrated it for many years (celebrations got fairly large while Obama was President, ours has been mostly historical exhibits, music, and various food booths). The June 19, 1865, date might make sense relative to Galveston, Texas, yet not so much anywhere else.
On January 3, 1861, just two weeks after South Carolina became the first state to secede from the Union, the state of Delaware rejected a similar proposal and did not secede.
Lincoln ordered the Emancipation Proclamation on September 22, 1862, to be effective January 1, 1863. The proclamation only applied to the 10 states in rebellion (secession), and not to other states and otherwise exempted areas. The proclamation was not enforceable in areas in rebellion until Union forces gained control of those areas (freeing the slaves as they went).
The war ended in Spring, 1865. Robert E. Lee surrendered the last major Confederate army to Ulysses S. Grant at Appomattox Courthouse on April 9, 1865. The last battle was fought at Palmito Ranch, Texas, on May 13, 1865. And, that last battle was actually a rebel victory.
The Thirteenth Amendment to the United States Constitution abolished slavery and involuntary servitude, except as punishment for a crime. The amendment was passed by Congress on January 31, 1865, and ratified by the required 27 of the then 36 states on December 6, 1865, and proclaimed on December 18, 1865.
Delaware, for example as a state not in rebellion (did not secede), while subject to the general ratification of the 13th Amendment in December 1865, was among the last to actually ratify the 13th Amendment, in February 1901, more than 35 years after the end of the Civil War.
Anyway, no problem with the celebration of freedom, yet why June 19th? Again, it is pertinent to Galveston TX, though slavery was ended per the wartime proclamation wherever the Union troops gained control (various dates from 1863 thru 1865), and slavery (and other indentured servitude) was still legal in some other places, unless/until ended by the actions of those individual states, or until the ratification of the 13th Amendment in December 1865.