The midnight sun is a natural phenomenon which means that the sun does not set during the summer months. In other words, given fair weather, you can see the sun for a continuous 24 hours.
The duration of the midnight sun depends on how far north you are. At the Arctic Circle in the county of Nordland, you can see the midnight sun from 12 June to 1 July, at the North Cape in Finnmark you can see the sun from 14 May to 29 July, and at the North Pole the sun does not set for six months.