Hello jiaruchan, I'm not a teacher, but I hope my advice helps!
A weekend is considered to be Saturday and Sunday in American culture, so it wouldn't be normal to wish someone a good weekend when they're already in the weekend. However, it is acceptable to say, "Have a good rest of the weekend!" Or you can say something else like it.
I hope this helps.
Thank you. Do you mean it is quite different from 'have a good day' said during the daytime when people are already there?
I'm not a teacher.
Well, I suppose. That would depend on the circumstances. Let's say a group of students has just arrived at a camp for the weekend. It's Saturday morning, and the instructor wishes the group to have a good weekend. But if it's Saturday evening or night, it's not normal to wish someone a good weekend. Even in the middle of the week, I've heard people say "have a good rest of the week."