It's Saturday or It's on Saturday?

Hey guys, I have a doubt. What is it correct (see below)?

a) It's Saturday

b) It's on Saturday

There is still more this:

a) It's Halloween

b) It's on Halloween

What's it correct between these setences?

I thought in this:
It's Saturday (É sexta-feira)
It's on Friday (É na sexta-feira) - For example, answering the question: When is the party?

Have a good day everyone!