The only tradition any of us has to worry about is whether to invite Uncle Joe in your Christmas card group photo. The cat-like people of the Purge Planet has way bigger issues and a higher body count.
For thousands of years, the upper class citizens of the planet has designated one night a year to pit the poor class against each other in a murderous spree known as the Purge. The village folks are convinced that getting all their violent aggressions out during this time will keep them passive until next year. To Rick, this is gleeful entertainment; for Morty, it's a barbaric custom.
When Morty hears the cries of a distressed damsel, he convinces Rick to save her. The girl, Arthricia, is grateful, but quickly steals Rick's spaceship, stranding the two. Rick and Morty gains shelter from an old man in a lighthouse who refuses to participate in the Purge. There, Rick crafts a beacon to receive a package he ordered while Morty endures the old man's terrible screenplays. When Morty offers some constructive criticism, the old man is offended and demands they leave his house. This prompts a frustrated Morty to murder him, thus associating him with the Purge.
The package arrives in time for Rick and Morty to defend themselves with, housing state-of-the-art armors with built-in weapons. They waste no time killing anyone in their paths until they find Arthricia. She claims to have stolen the ship to stop the Purge once and for all, but Morty doesn't believe her. Rick knocks his grandson unconscious before the latter has a chance to kill Arthricia. He then assists her in executing all the rich citizens, thus ending thousands of years of needless bloodshed.
But history is a dangerously fickle thing; after Rick and Morty leaves, the villagers try to rebuild their society, but end up arguing amongst each other on what roles they should play. A man offers a suggestion where they take one night off to get out their aggression so the rest of the year will be peaceful. Like I said, fickle.