Tomorrow is the Juneteenth federal holiday. No mail, no courts, banks are closed. I will not be working. Working on this holiday is a covert way of saying it isn't an important holiday. If your boss says you have to work, especially in a non-urgent job, gently reply (this is NOT legal advice) something like, "Oh, I knew you were an unapologetic capitalist, but I didn't know you were also a racist." Consider adding "I'm getting holiday/overtime pay, right?"