FIND IT ON THE NORWAY MAP
Due to the fact that the Earth spins around the sun at shifting angles throughout the year, some areas of the globe has periods during summer when the sun never sets and periods during winter when the sun never lifts above the horizon.
In general, this phenomenon increases the further north (or south) you travel. On the poles "a day", if defined as the time between sunrise and sunset, will last for months. So will the "night" during winter.
The most southern latitude where the midnight sun (and midday darkness) can be observed, if only for one day a year, is called the Arctic Circle. It's about 66°33'39" degrees north, but exact line varies from year to year.
To find the approximate location of the Arctic Circle on the Norway Map, zoom in on "the neck" of Norway and find the towns Bodø and Rana. Zoom in even more and find the Island of Rødøy. The Arctic Circle travels just a bit south of that island and inwards at the same latitude crossing the Svartisen / Saltfjellet area.
If you travel north of this line you can play tennis in broad daylight in the middle of the night (bring shades if you'd like to sleep). In winter you may get lucky and catch a glimpse of the Northern Lights instead.