This is the 6691st most frequent Swedish word.
månadens
"Month's" (possessive form of "månad," meaning month).
The term 'månadens' is used here to specify an attribute belonging to this month, defining the 'offer' as relating to the current month.
Månadens erbjudande är en halva priset på alla böcker.
This month's offer is half price on all books.
The word 'månadens' specifies possession or association with the current month, referring to an exhibition held during this time.
Vi besökte månadens utställning på konsthallen.
We visited this month's exhibition at the art gallery.
In this context, 'månadens' serves to qualify 'the best movie', indicating that it is considered the best of all released during the current month.