In terms of the Earth's orbit around the sun, the longest season is the (which is simultaneously the Southern Hemisphere's Winter). Duration: Approximately 93.65 days .

The length of a season can be measured in two main ways:

The reason summer is the longest season lies in the Earth's axial tilt. The Earth's axis is tilted at an angle of about 23.5 degrees relative to its orbital plane around the Sun. This tilt causes the amount of sunlight that reaches the Earth's surface to vary throughout the year.

Winter is December through February; Spring is March through May.

In the world of professional athletics, the "season" is defined by the grind of competition rather than the tilt of the Earth. Meteorological Versus Astronomical Seasons | News