what to wear at -20 degree celsius?

Better to wear layers to trap in body heat topped by a warm, waterproof jacket with a hood, also warm hat scarf and gloves. Warm trousers. Sensible winter boots with a good tread to get a grip in snow and warm socks. Wear thick socks when you go to buy the boots, or they could end up being to tight.

Good luck on your trip

Beulah

That is -4F and remember that doesnt count the wind chill. If the wind blows at all, it can drop the how the temperature feels and affects your skin dramatically. You need to dress in layers, shirt, heavier shirt, light jacket and heavy jacket. Clothes that block the wind, and dont stay wet. Socks are important so you dont get frostbitten feet. Hats and gloves also. It really depends on how long you will be outside. But skin can freeze in a short time with below zero temps and the wind blowing.

well, 0 degrees celsius is freezing point (32f). So that is basically single digit weather in Farenheit, so cold

A winter jacket, if you still think its cold put on a sweater under the jacket.

In my view over coat and warm trouser.