For the outer layer, do not go with synthetic stuff, despite what people will tell you. An oversized Icelandic wool sweater or similar will catch snow and wind much more effectively (and dry in no time when indoors.

… A sunny disposition?

Layers - get a good base layer, merino wool is the gold standard for these. You also want to get a neck gaiter, plus some merino wool socks, and some kind of cap. These will keep you warm and dry throughout most cold weather. Add more layers on top when outside, and remember to remove layers proactively if you feel yourself starting to sweat - it’s never a good idea to be wet in cold weather. The base layer will make physical exertion a pretty hot affair quite fast, so you’d be surprised how fast the sweat starts pouring.

If you can, ask someone already there. Depending on the exact type of work, they may have very specific recommendations, and they’ll have a better idea of what the climate is like than we will.

In general, you’ll want to be able to layer clothing. Start with the base layer, whose job is to be wicking away sweat.

Mid layers are for insulation, and it would be prudent to get one light sweater and one heavier sweater, maybe a third or whatever. The idea being you can increase your mid layers to stay warm but not too warm as necessary. Same for pants. Around here, I usually go for a tight base layer, a loser waffle-weave longjohns and shell blouses into boots.

Keep in mind, that layers being worn to the outside should be looser so as to not compress layers being worn closer to the body.

Patagonia makes some good, hard wearing stuff as a general brand to check out, but there far from the only one.

Hats and gloves are important, too and for gloves I’d consider getting mittens at least as one option and maybe lighter fingerless gloves to wear inside. (Or lighter gloves. Especially if it’s possible you’ll need manual dexterity)

Also, bring a book or something to read, and plenty of snacks that don’t necessarily freeze. Hydration is also important and illumination.

Try to stay away from caffeine, among other things it also constricts the blood vessels limiting circulation in your extremities.

Layers are the way to go.

1. Long Underwear, a long sleeve breathable undershirt, good WOOL socks
2. A sweater or other medium-thickness top garment.
3. Waterproof snow pants and coat.

Good boots, preferably waterproof if you’ll be in the snow a fair amount. Definitely good so you have better grip on snowy/icy ground. Thin “liner” gloves and thick waterproof gloves to go over those. A warm hat that covers your ears (I really appreciated my trapper hat when I went to Tromsø).

Avoid cotton. Cotton is horrible at insulating if it gets wet, as opposed to wool which can keep you warm while fully drenched.

Northern scandinavian, here.
This depends on where specifically. Near the ocean it’s not that cold at the moment (well, depends what you’re used to, I guess. I get around fine with t-shirt and a light jacket. Temp varies between 5C and 15C nowadays.

But in general, I suggest you bring a sweater , some woollen socks, and a jacket that can handle rain/slush/snow. Staying dry means staying warm. Dressing with layers is better than one thick coat.

Snow will set in soon, though, so if you’re going to be working outside I recommend some gloves. Preferably two pairs, so one can dry off while you’re wearing the other pair.

May I ask where specifically you’re going? Coast vs inland makes A LOT of difference. Humidity can make 5C near the coast feel colder than -20C inland. Plus wind isn’t as much of a factor far away feom the ocean.

Layers are important for your clothing. You don’t want to get too hot if you’re working outside. Breaking a sweat will make you freeze, so you need to be able to adjust throughout the day.

You going sometime soon? Around December the temps tend to plunge and Jan-Feb can be really cold, somewhere around -40C, potentially with some wind and blowing snow making it feel even colder. Highly recommend good outside set, with winter boots, gloves and the rest of clothing meant for that sort of weather, especially if you expect you would need to be outside for a while. I like the overall type pants and winter rubber boots big enough to fit wool socks in myself. :P Might be difficult to buy good quality stuff if you live somewhere much warmer. Probably worth asking whoever is hosting you for some help there. They might even have extra sets available already. Houses up there are typically very warm, so nothing special needed for inside use. Otherwise maybe a camera that is good in low light if you like to take pictures. Should get some good northern lights and stuff if you’re up there a whole month.

Your wife and young son, plus a sturdy axe.

money

spikes, wool, layered clothing, reflective vest or other reflective gear…

Is it innland, hige land or coastal?

I would how you have a contact they’re who can answer these questions