Polar night

Phenomenon where the Sun remains below the horizon for more than 24 hours

Polar night is a phenomenon that occurs in the northernmost and southernmost regions of Earth when the Sun remains below the horizon for more than 24 hours. This only occurs inside the polar circles. The opposite phenomenon, polar day or midnight sun, occurs when the Sun remains above the horizon for more than 24 hours.

From Wikipedia, under CC BY-SA. More on occurri.