“The myth we believed in was becoming reality,” radical activist Abbie Hoffman wrote in his 1980 autobiography, looking back on the 1969 Woodstock festival. “We were not alone. Acres of freaks. No cops. . . . The greatest musical gathering in history taught us we could be together.” Since 1989 the Mary-Arrchie Theatre Company has sought a similar sense of community with this annual Woodstock anniversary celebration, a weekend-long marathon of experimental performance and schmoozing fueled by an anything-goes aesthetic and sleep deprivation. The festivities begin Friday at 3 PM at Daley Plaza, where actor-director Richard Cotovsky adopts the persona of Hoffman to lead a march from the Loop to Mary-Arrchie’s Uptown venue. The entertainment begins in earnest at 7 PM Friday and continues almost nonstop through midnight Sunday. A schedule, subject to change, is posted at www.maryarrchie.com. Among the likely highlights are two Mary-Arrchie productions: Arlene Cook’s Gas Mask 101, a wistful, well-observed portrait of college guys fretting over the draft in the era of guilt-free dope and sexually segregated dorms (10:05 PM Friday and 11 PM Sunday), and Gardner McKay’s romantic drama Sea Marks (4 PM Sunday). Other participants–some 50 ensembles and solo artists–include A Red Orchid Theatre, the Factory Theater, Suspicious Clowns, Dana Block, Brian Kirst, and Steppenwolf ensemble member Robert Breuler, whose new play The Pretzel Sisters will be staged by Breuler’s wife, actress Suzanne Petri. 8/19-8/21: Fri 7 PM through Sun midnight. Mary-Arrchie Theatre Company, Angel Island, 735 W. Sheridan, 773-871-0442. $10 for a one-day pass (come and go at will); $25 for a weekend pass.