"It is September."
Well in this case, I wouldn't say September is considered a subject in the sentence, it's more of a predicate. Take a look:
-What month is it? -Τι/Ποιος μήνας είναι;
-It is September. - Είναι Σεπτέμβριος.
Almost like you're answering "The month is September." So the month looks like the actual subject ^.^ (These "month rules" in Greek are really weird, but they are not too complicated. Soon you'll get used to it, and the cases where you'll be a bit confused are really, only a few.)