Tomorrow is my day off? or Tomorrow is my off day?

Is this correct?

Tomorrow is my day off?
or

Tomorrow is my off day?

The situation is that i don’t have to work tomorrow. I want to use day off … thanks

Either can be used, but the first is more commonly used.

Use the first.
an “off day” can be a day when you’re not quite right. Jim is having an off day, today.

“day off” as a phrase means a day when you’re away from work

… just be noted that the plural form of “day off” is “days off”…

Ex: I’ll take some days off before my wedding.