Have been once to north Finland, Saariselka, flew by Jet2 to Ivalo (was a charter for 5 day Santa specials) arrived day before Christmas Eve.
Just got about 3 hours of greylight (a sort of twilight), sunrise was 2 weeks away. Definitely worth doing as snowy wilderness is amazing. Especially if get couple of hours driving husky sledge, and/or skidoo safari
Haven't been that far north in mid summer, but been close enough that when the sun set, never got totally dark, and sun was rising again less than 3 hours later. Basically further you get from equator twilight is longer.
Have been to Alaska (and train from Fairbanks to Anchorage) with overnight stop at Denali. That was early September, Autumn is only about a week long, (leaves had gone yellow, and snow was starting to settle on hills). Daylight rate changes rapidly, was about 9 minutes less each day in September.
Definitely would recommend a winter trip, but if do it late Dec, early Jan, just accept you will not really see anything because of the dark 20 hours a day, but take a head torch and enjoy snow when dark.