Day off / Off day
Different meaningsA day off is a non-working day; an off day usually describes unusually poor performance or condition.
Day off
Use when
You mean a day when someone is not working.
Example
“Tomorrow is my day off.”
Off day
Use when
You mean a day when someone's performance, mood or condition is worse than usual.
Example
“The presenter was having an off day.”
Quick check
on off day tomorrow, my off day when day off is meant
Usage note