One full revolution of the earth is divided into these four different seasons: Summer, Autumn, Winter, and Spring. The reason we have these seasons is because earth is not perfectly aligned to the sun as it rotates on its axis around the solar giant.

The cycle of seasons is caused by Earth's tilt toward the sun. The planet earth rotates around an (invisible) axis. At different times during the year, the northern or southern axis is closer to the sun. During these times, the hemisphere tipped toward the star experiences summer, while the hemisphere tilted away from the sun experiences winter. Our planet isn't perfectly upright when it spins. Thanks to a few collisions during its formation, the Earth is tilted at an angle of 23.5 degrees.

This means that as the Earth takes it annual trip around the Sun, different areas of the planet face the Sun more directly during their daylight hours at different times of the year.

The tilt also affects the daily amount of light — without it the whole planet would have 12-hour days and nights every day of the year.
The imperfect tilt of the earth to the sun is responsible for the seasons of the year.

Summer: June to September
Autumn: September to December
Winter: December to March
Spring: March to June

Further reading
https://www.timeanddate.com/calendar/aboutseasons.html
https://www.livescience.com/25202-seasons.html
http://www.abc.net.au/news/science/2017-09-01/seasons-and-their-changes-explained/8858776

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one generation of the sun is 365 days

one complete generation/revolution of the sun is 365 days.

One generation of the sun is 365days, this count begins at the birth of the sun on the 25th of December.

One revolution is one year, one Roshhashanah to one Roshhashanah is one generation

One revolution =one generation =one year

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