Page "Arctic Circle" Paragraph 4

The Arctic Circle marks the southern extremity of the polar day ( 24-hour sunlit day, often referred to as the midnight sun ) and polar night ( 24-hour sunless night ).

North of the Arctic Circle, the sun is above the horizon for 24 continuous hours at least once per year ( and therefore visible at midnight ) and below the horizon for 24 continuous hours at least once per year.

On the Arctic Circle those events occur, in principle, exactly once per year, at the June and December solstices, respectively.