Please explain why in the French phrase "Les vendredis soir", "soir' is an adverb?

Please explain why in the French phrase “Les vendredis soir”, "soir’ is an adverb?, which for reasons I don’t understand “les vendredis soirs” is incorrect but “les vendredis soir” is.

I would guess there’s an implied contraction “les vendredis [au] soir”. I would assume that, depending on context, the whole thing counts as an adverbial phrase, the verb being whatever happens on those Friday evenings

That makes sense PatrickLondon.Thanks.