The Hare Club For Men is an organization which was featured in the South Park episode "Fantastic Easter Special".
The organization keeps and passes down the secret of Easter. In "Fantastic Easter Special", it is revealed that the secret of Easter is that the first Pope, St. Peter, was actually a rabbit. Leonardo Da Vinci had wanted to paint St. Peter as a rabbit in The Last Supper, but the Vatican opposed. So, Da Vinci painted St. Peter as a man, but left clues in his painting that he had in fact been a rabbit, just as Jesus had intended for the Pope to be.
Since then, the Hare Club For Men has been painting Easter Eggs in order to keep the secret of Easter alive and protecting the descendants of Saint Peter.
Most recently, the Hare Club was charged with the protection of the rabbit Snowball, who after Jesus' defeat of Pope Benedict XVI and Bill Donohue, becomes the newest Pope.
In "Fantastic Easter Special", Jesus reveals that he had wanted the Pope to be a rabbit because humans are intolerant and corrupted, while rabbits are pure.
Stan's grandfather, Marvin Marsh, and father, Randy Marsh are members of the Hare Club For Men. Stan later becomes one too in the episode "Fantastic Easter Special." Randy claims that the men in the Marsh family have been in the club for generations.
When they were bringing Snowball out, all members were singing an Ode to the Rabbit in Latin, to the tune of "Here Come Peter Cottontail". The lyrics are not a direct translation of the original Easter song, but extol the virtues of St. Peter Rabbit.