Midnight sun is when the sun for 24 hours does not sink below the horizon. This occurs in polar regions for all or parts of the summer, depending on how close to the poles you are. The period with midnight sun is the longest at the actual poles (north and south), where you can see the sun 24 hours a day for about 6 months (unless there are too many clouds). Regions with midnight sun have accordingly long periods with winter darkness.
Photo: Children playing under the midnight sun in Tromsø, Norway. Photo: Frank Meissner.