Grammar it or that

I am going to see my sister today. It is going to be a nice day.
Yesterday I went to see my sister. That was a nice day.

It depends on which day you are talking about. (Past, present, or future)
You could say "I'm gonna go see my mother next Monday. That's gonna be a nice day."

Is it correct? I am going to see my sister today. It is going to be a nice day.
{Past) Yesterday I went to see my sister. That was a nice day.
Thanks very much for helping me :-}

Yes, that is correct. You are very welcome.