All acoustic musicians invited to jam with the members of this award-winning Swedish traditional music quintet, which appears at the Ark tomorrow night (see Nightspots). Jaerv’s members play fiddle, nyckelharpa, drums, guitar, and flute. The evening begins at 6:30 p.m. with a potluck dinner. Listeners welcome. 7:30–9:30 p.m., Gretchen’s House, 2340 Oak Valley Dr. Free. [email protected], jaerv.com.