Seasons in Earth are beautiful...we all experience different seasons during different parts of the year.
Why we have different seasons? It is because of two reasons:
1) Axial Tilt of Earth: Earth is not just rotating itself and orbits around the Sun. It does so with an axial tilt. That is at all times as the Earth rotates around the Sun, earth has a tilt of 23.4 degrees with its orbital plane.
When it is tilted, different hemispheres of the Earth - Northern and Southern halves of the earth receive different amount of sunlight.
2)Earth is orbiting + Axial Tilt: In the same orientation Earth rotates around the Sun in an elliptical orbit. Because of this travel, proximity of the poles to the sun changes all around the year.
Because of the above two reasons Earth experiences the following celestial events every year - giving us the seasons every year.
An equinox happens when the plane of Earth's Equator passes the center of the Sun. Equinox is when the Earth's rotational axis is tilted neither toward nor away from the sun. The equinox occurs at the point at which the suns path or the eccliptic passes the celestial equator.
During Equinox nearly equal amount of daylight and darkness received all over the Earth. Equinox occurs twice in the year - Once in March and Once in September. The March Equinox occurs on any day between 19th March to 21st March. The September Equinox occurs on any day between 21st September to 24th September.
For the northern hemisphere March Equinox starts the spring season and the September Equinox starts the fall season. For the southern hemisphere September Equinox starts the spring season and the March Equinox starts the fall season.
On June 21 the Sun is directly overhead at noon on the Tropic of Cancer - which is called June Solstice. On December 21 the Sun is directly overhead at noon on the Tropic of Capricorn - which is called December Solstice. The June solstice marks beginning of summer in Northern Hemisphere and beginning of winter in Southern Hemisphere. The December solstice marks beginning of winter in Northern Hemisphere and beginning of summer in Southern Hemisphere.