Joe Segal, the venerable producer/promoter of the Jazz Showcase, is billing this concert as "The Jazz Event of the Year!" Segal was never one to shy from hyperbole; this time, though, his description may well fill the bill. To celebrate his 45 years of presenting "sessions"--Segal's word for almost everything from a pickup jam to a full-blown concert such as this--he's gathered a sparkling assortment of musicians who've worked at his clubs over the last four decades, most of whom have strong Chicago roots. The cast includes vocalist Joe Williams; saxophonists Stanley Turrentine, Lee Konitz, and Johnny Griffin; multiinstrumentalist Ira Sullivan; trumpeter Art Farmer; a raft of pianists headed up by Billy Taylor and Ramsey Lewis; and others who, despite their occasional differences with Segal, have come to admire and respect him for persevering in the name of the music he loves. True to the original concept of his sessions at Roosevelt University--where he became an amateur impresario in 1947, while still a student--Segal plans to mix and match the musicians, featuring each on several numbers and still finishing up in two and a half hours; as he points out, when you do it for 45 years, you get pretty good at moving people on and off a stage. A quartet of guest emcees will join in the honors; all proceeds will benefit the scholarship fund of the Chicago Musical College at Roosevelt. (Segal will also be producing some far more informal "sessions" Tuesday at his alma mater, for the 1947 price of two bits.) Monday, 7 PM, Auditorium Theatre, 50 E. Congress; 341-3780 or 902-1500.