You'd normally use "day off", to mean a usual workday, you are not working. A day off work. "I'm having a day off." If you say, "I'm having an off day," it means something completely different. This is a day when you don't feel properly focused or well, and maybe you're having a run of bad luck and are in a bad mood. A sportsperson who usually plays very well might say, "I'm having an off day," to explain a poor performance.