The かなあ shows that it's something you long for. As you probably know, the particle か shows that it's a question. People often say -かな add some feeling into the question. and the long あ in -かなあ, indicates that you wish for it, of ask for it.
I think you could also translate the sentence as: "Can't it become Friday soon?" with a kind of wishful tone.
This is a very difficult thing to explain, and I think it might be easier to learn by doing in this case.
Also, i'm not a native speaker.. but i hope my explanation helps anyway! :D