Why is the length of day different in different seasons?
The length of the day changes from season to season due to the Earth's axial tilt.
The Earth's axis of rotation is inclined at
Let's see what happens during a year in the northern hemisphere.
At the vernal equinox, which is around 20 March, the Earth's axis of rotation is perpendicular to the direction from the Earth to the Sun. The Sun is directly over the Equator and the days and nights are equal length everywhere.
As time passes the Sun's position moves North and more of the Northern hemisphere is in direct sunlight. The days get longer. At the June solstice, the Sun is over the Tropic of Cancer and the days are their longest in the Northern hemisphere.
After the solstice the Sun's position moves South until it is over the Equator at the September equinox. The days get shorter in the northern hemisphere and the days and nights are equal length at the equinox.
After the equinox the Sun's position moves South of the equator and the days in the northern hemisphere get shorter than the nights. The shortest day is at the December solstice.
Finally the Sun's position starts moving North and the northern hemisphere days get longer until the Sun is again over the equator at the March equinox.
The reverse happens in the southern hemisphere.