I'm guessing predicate nominative because month is renaming September.
In this context, it would be who. One would use whom to sound smart, but if used incorrectly, sounds ignorant.
A antonym for "old fashioned" is new, because when something is old fashioned, it's usually old, and doesn't look in a good state, while something is new, it looks better than something old fashioned, or up to date
I hope this helps! :>