A poster promoting one of Salsedo Press’s Cinco de Mayo blasts–which are famous in Chicago’s lefty ghettos–shows a few of the world’s best-known politicos shaking their booties on a fire escape high above Chicago’s skyline. Emiliano Zapata looks stern. But an olive-drab Daniel Ortega is getting down, and a dapper Nelson Mandela has his fists raised above his head in funky triumph and is wearing a pin that says “Free Beer.” Harold Washington’s grinning face forms a cloud on the horizon.

Best of Chicago voting is live now. Vote for your favorites »

Salsedo Press–a worker-owned, for-profit business that prints everything from magazines to wedding invitations–was founded as a workers’ cooperative in 1969 by six University of Illinois students who were losing their battle to keep local printers from censoring their antiwar newspaper, the Walrus. When U.S. involvement in Vietnam cooled in 1973, student activism on the Urbana-Champaign campus did too.

The next party was tied to May Day, says Burke, “because May Day originated largely in Chicago with the movement for the eight-hour workday and the aftereffects of Haymarket.” Soon the parties became annual bacchanalia for hundreds of progressives, sometimes drawing nearly 1,200 to hear world music, drink free beer, and make leftist love connections. “The party kind of took on a life of its own,” says Burke, who knows two married couples who met at one of the parties. Politicians, petition-wielding organizers, and famous-activist impersonators have also courted one another around the makeshift dance floor, beer tables, taco concessions, and plastic-draped printing equipment.