The planet Saturn was named after the Roman god of the same name, but any symbolic similarity ends there.
For Saturn, the god, was feasted with unrestrained merrymaking, and we still recall these riotous feasts in our term Saturnalia.
On the other hand, Saturn, the planet, was the seventh and most remote planet known to ancient astronomers, and this remoteness was presumed to confer a cold and gloomy attitude upon those under its influence.
In sharp contrast to the gay Saturnalia, then, we have the adjective saturnine, “gloomy, dull.”