Midnight sun is a phenomenon that exists north of the Arctic Circle and south of the Antarctic Circle. It means that the sun, on parts of the year, is visible the whole night long. The sun above the horizon at midnight in the arctic or Antarctic summer.
There are no people living permanently south of the Antarctic Circle, but in Alaska, Canada, Norway, Sweden, Finland, Russia, and some other countries it is possible to watch the Midnight sun in the summer.
The opposite of the Midnight sun is the Polar night.