There is a movement afoot to make Juneteenth a National Holiday. People likely think this is free, and is just a nice way to show African-Americans that we care about them. Who could be against that? You wouldn't want to be against that, would you? That would be unkind, impolite, and racist.
Articulate what Martin Luther King Day is for. The first meanings of that verb are "utterance," "putting into clear words," and that's what I mean. If you want Juneteenth, you should first have to put into words what MLK/Civil Rights Day is for, not just think about them vaguely and have a feeling. Only then can you go on to describe how Juneteenth is different and brings something new to the table.
I'll just wait here while you scratch some things on the page and imagine delivering those words before an audience. They have contests for that, don't they, asking schoolchildren to write speeches about what that holiday is about? What do they say, do you think?
When you have finished that, scratch down some percentages of what a new federal holiday will cost businesses and governments which would then have to pay people to stay home, or at minimum pay them a higher wage. Describe to me where that money will come from. As a starting point, people work 5 days/week for 52 weeks, minus ten days vacation minus fifteen holidays minus sick days - about two weeks. Call it 225 days a year. Back of the envelope is fine.
Now remember that this will feel good to do but have only psychological effects on people who really dig this stuff. There will be no improvement in policing, or schools, or job prospects, or city infrastructure, or, well anything. Hispanics might rightfully wonder why they got left out. At least "Civil Rights" applies to everyone, at least in theory.