Unless you have an employment contract or union agreement that prohibits this action, it is legal. Otherwise, as an "at will" worker your employer can set the conditions of the workplace much as it sees fit (absent somefomr of legally actionable discrimination/retaliation). This includes having you come in to work on your day off. If you fail to do so you can be disciplined, up to and including suspension or even termination. In fact, you can be fired for any reason or no reason at all, with or without notice. That having been said, if you are a "non-exempt" employee (typically one who is paid on an hourly basis), then you must be paid for the time that you come in. Further, if that puts you over 40 hours for you work week, you must be paid OT for such additional time.