It was interesting to read that your "second-great grandfather" fought in the Corporal First Cavalry for the Union army with the roughly 79,000 black men and 2.4 million white men, who served in the Union Army. I also had an ancestor who fought for the Union sustaining terrible wounds at Gettysburg and managed to survive, but was crippled for the remainder of his life...
The victory which provided Lincoln the platform on which he delivered the Gettysburg address and put in motion first the emancipation of slaves in the South and later with the passing of the 13th Amendment, the end of legal slavery in America.
I support everyone learning how throughout history blacks and whites fought beside one another starting with the American Revolution up until the present. Perhaps more should be made of the shared history of all Americans on Veteran's Day and Memorial Day.
As for Juneteenth, I see it as a distinctly black holiday simply because it is not exactly relevant to American history, but more relevant to black history. I say that because it did not happen within the United States of America, it was not the day when the last slaves in what would become America were told they were free, and in fact, the slaves themselves were free as far as the United States was concerned after Gettysburg.
So, it seems more of a symbolic day picked by someone to serve as a reminder of how long the struggle to attain the promised equal rights first expressed in our Declaration of Independence took for slaves.
I do think July 9th would be a great national civil rights day, since on that day, the 14th Amendment made all persons born in the United States including former slaves US citizens entitled to protection under federal law. What a great "week" that would make... starting with the "promise" of people being equal and ending with the celebration that provides the legal foundation for every civil rights movement in American history.