The Antarctic Circle is one of the five major circles or parallels of latitude that mark maps of the Earth.
It is at latitude 66 degrees 33′ 39″ south of the equator (in 2000; like its northern counterpart, the Arctic Circle, the value is currently slowly decreasing over time, pushing the Antarctic Circle southwards with about 15 m per year).
For everywhere within the Antarctic Circle, there is at least twenty-four hours of continuous daylight on the Summer Solstice in December, and at least twenty-four hours of continuous nighttime on the Winter Solstice in June.
That is to say, one whole day during which the sun does not set, and one whole day during which the sun does not rise.
This is because the earth is tilted at a 23.5 degree angle, and during the winter solstice, the southern hemisphere is tilted away from the Sun, meaning that the antarctic circle is completely tilted away from the Sun, hence it experiences 24 hour nighttime, and vice versa.