Sunlight, a vital ingredient for life on Earth. However, due to Earth's rotation around the sun and its tilt, sunlight isn't the same throughout the year and therefore the sun's energy varies.
During the Northern Hemisphere's Summer, the Earth is tilted towards the sun on its axis, redistributing the suns energy.
The Arctic Circle marks the point where sunlight north of this line doesn't set for 24 hours. The home of the midnight sun. Hence the phrase "the longest day of the year!".