Employers get to decide what days are workdays and what days people may take off. In the U.S. there aren't any guaranteed holidays. In fact, except for a few jobs, there is no requirement for you ever having a day off. Think about all those retail stores that are open on MLK Jr. Day, Presidents Day, Good Friday, Easter, Memorial Day, etc.
I believe that employers should publish their policies including which days are allowed holidays. But not all do. I also believe that employees should read those policies. But not all do.