What becomes a legend most? Longevity helps; so does clean living (even if it only comes in occasional and limited doses). But most of all, you need the kind of artistic charisma that poured out of Anita O’Day when she was discovered in Chicago in the early 40s, a quality that illuminated her years with Gene Krupa and then Stan Kenton; anyone who’s ever seen her under that hat and behind those elbow-length gloves, as she appeared in the classic concert film Jazz on a Summer’s Day, needs no further introduction to star quality. O’Day’s radiance sprang from her music. Her singing was fearless (and still is, though less so); in her scat improvisations, she has always taken the same risks as the greatest instrumentalists, and more risks than the mass of mediocre ones. She’s slowed down some, of course–she’s almost 71–but she can still pull out some stops and construct a solo that practically defines artistic bravura. She’ll have plenty of like-minded help this weekend, with the California saxist Gordon Brisker, the unsinkable Chicago violinist Johnny Frigo, and the Methuselan piano master Art Hodes all taking turns; Sunday also brings a matinee set by Louis Bellson’s big band, as part of a weekend jazz party that also features dancing, jazz videos, and a Sunday jazz brunch. Tonight and Saturday, 9 and 11 PM, and Sunday, noon-2 PM and 4-8 PM, Joe Segal’s Jazz Showcase, Blackstone Hotel, 636 S. Michigan; 427-4300.