Just to clarify, Finnair (AY) only use their 757s on IT charters (and a few charter-like schedules) with charter-pitch seating, it is quite a different sub-fleet from their trunk services across Europe which they use 321s/320s on, with normal scheduled-type seating, business class, etc. For many years recently Air Scandic in the UK used this spare summer 757 capacity, but they have gone now.
Just like the Canadian charter operators they do summertime transatlantic charters with them to Toronto etc, a market which peaks in the opposite part of the year to their Finnish winter sun holiday business. Actually not just sun, they surprisingly do ski charters to the Alps as well, Salzburg being a particular favourite. Plenty of snow in Finland then, of course, but not a lot of good mountains to go down and very little daylight hours anyway !