What is the polar night, and what is it like to visit Vadsø then?

Short answer

The polar night (mørketid) is the period when the sun does not rise above the horizon — in Vadsø from 25 November to 17 January. The town is still fully active, just in blue light. The long, dark evenings give some of the best conditions in Norway for the northern lights, and you can join a guided northern-lights tour. See Northern lights in Varanger and Inspiration.