The semantics are slightly different. I'm not sure how best to explain it, but I see it kind of like "today" is the subject noun, and by stating "it" as another (identical) subject, you are reinforcing the statement of the sentence. Adding "it" probably helps with adding tonal inflection when trying to get a message across. I think it's just an unnecessary extra word, but both sentences do carry the same meaning.
Please correct me on this or expand further! I'm told I'm well spoken in English, but during school we weren't taught about the structure of the language at all.
In charnfield's sentence, "today" is not an "identical subject," or a subject at all—it is an adverb. It does not answer the question of what is not Thursday—that is "it"—but rather when "it" is not Thursday. You can break down the construction as follows: "It is not Thursday." "When is it not Thursday?" "Today. Today, it is not Thursday."
A parallel construction might be "Today it is not a good idea." This does not mean that "today" is not a good idea, but rather that "it" is not a good idea today: tomorrow, it might be.