* Seasons are defined by the tilt of a planet's axis. Planets with a significant tilt experience seasons.

* The length of a year is determined by the time it takes a planet to orbit its star.

While there are planets with extremely long years (like Pluto, which takes 248 Earth years to orbit the sun), the length of a season isn't directly tied to the length of the year.

Therefore, no planet in our solar system has a summer that lasts 21 Earth years. It's possible that some planets around other stars might have such long seasons if they have a very specific combination of axial tilt and orbital period.