The Arctic Circle is located at 66.5 degrees north latitude and the Antarctic Circle is at 66.5 degrees south latitude. These lines mark the northernmost and southernmost places where the sun can still be seen above the horizon on the solstices.
Interestingly, for the duration of a hemisphere's winter, the season when it is tilted away from the sun, there is complete darkness above its polar circle. At the same time, however, it is summer in the other hemisphere, as it is tilted toward the sun. Therefore, within its polar circle, the sun remains in the sky for an entire season.