Definition

Day and night happen because Earth spins: the half facing the Sun has daytime, the half facing away has nighttime, and as Earth keeps turning every place gets a turn in the light and a turn in the dark. Earth rotates on its axis once every $24$ hours, so at any moment half of Earth is lit by the Sun and half is in Earth's own shadow, and the boundary between them, the terminator, sweeps westward across the surface as Earth turns eastward. The length of daylight at a given place depends on latitude and season because of Earth's $23.5^\circ$ axial tilt. Quantitatively, on a date when the Sun's declination is $\delta$, daylight at latitude $\phi$ is set by the hour angle of sunrise, $\cos H = -\tan\phi \tan\delta$, with daylight lasting $2H$ converted at $15^\circ$ per hour; when $|\tan\phi \tan\delta| > 1$ there is no sunrise or no sunset, producing polar day or polar night poleward of $66.5^\circ$. Atmospheric refraction lifts the Sun about half a degree at the horizon and twilight extends usable light beyond geometric sunset, so actual daylight is a few minutes longer than the formula gives.