AKA White nights, polar night, midnattssol
What is it? When the sun stays above the horizon 24 hours a day, remaining visible even at midnight.
Where? Within the Arctic and Antarctic circles, though also occurs up to 90km outside circle regions.
The midnight sun means twice as much holiday, sort of those long hours of daylight allow you to hike, sail, wildlife watch and explore until the wee hours; many travellers report needing less sleep when surrounded by all this illumination.