The summer solstice, around June 20-21, marks the longest day in the Northern Hemisphere. The event occurs due to Earth's axial tilt of 23.5 degrees, with the North Pole closest to the Sun and the Sun directly over the Tropic of Cancer at 23.5°N.The summer solstice, around June 20-21, marks the longest day in the Northern Hemisphere. The event occurs due to Earth's axial tilt of 23.5 degrees, with the North Pole closest to the Sun and the Sun directly over the Tropic of Cancer at 23.5°N.