The Arctic Circle is one of the five major circles of latitude that mark maps of the Earth. In 2009, it is the parallel of latitude that runs approximately 66° 33′ 39″ (or 66.56083°) north of the Equator. The region north of this circle is known as the Arctic, and the zone just to the south is called the Northern Temperate Zone. The equivalent Polar circle in the Southern Hemisphere is called the Antarctic Circle.
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 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.
This picture was taken at 11:00pm, the sun never setted down.