One of the most common grammar mistakes I seen online is the confusion between "DAY OFF" and "OFF DAY"
I honestly get annoyed about the confusion. So... I AM GONNA MAKE THE WHOLE WORLD UNDERSTAND THE DIFFERENCE!
Let me put it as easy as ABC...
"DAY OFF" means "taking a break" or "holiday" (in some context)
eg : Today is my day off and I am going to sleep whole day.
"OFF DAY" means "having a bad day" or saying everything is not going according to plan.
eg : Leo Messi is having an off day, he seems to missed all his opportunity to score.
So.. there you have it.
I hope you understand.