In early to mid-September, the evenings are cool enough that most locals plan to need a sweater or light jacket even when going out in the early evening. But this could turn out to be an unusually warm September, so who knows for sure; and then it's a question of how to deal with the windows open to cool the room off. If upgraded to or having booked the Attic for July, I would perhaps even want a downgrade; in September, probably not. But even in December, if the hotel has the heat on, it can have me wanting to use a fan and open windows even at the relatively nearby Reisen in Stockholm.