It's x is: Qual utilizar?

Hello there

I was writing my status on facebook and I had a huge doubt about the pronoun it, what would be the correct sentence: "Tonight, it's Harry Potter night!" (Means Im going to watch Harry Potter tonight), or "Tonight is Harry Potter night!" and yet, should it be Harry potter's night or just Harry Potter without apostrophe?

I also need to know why? Automatically came "it's" instead of "is" to my mind but I didn't know if it's incorrect.

Thanks