On each Tropic, the sun is directly overhead at noon once a year. When this happens on the Tropic of Cancer, it is summer solstice in the northern hemisphere. When on the Tropic of Capricorn, it is winter solstice.
On the Equator, the sun is directly overhead on two days. These are the spring equinox in March and the autumn equinox in August. Across Earth, day and night are of equal length on these two dates. On the Equator, day and night are equal every day.
The planet’s axis of rotation is tilted at an angle of 23.5°C. This tilt — combined with factors such as Earth’s spin and orbit — leads to variations in the duration of sunlight on any location on different days of the year.
At latitudes of 23.5° (matching the tilt) are the Tropics of Cancer and Capricorn, north and south of the Equator. At 66.5° (or 90° minus 23.5°) are the Arctic and Antarctic Circles, north and south. It is at latitudes higher than 66.5° (in either direction) that days of constant darkness or light occur.