This concert celebrates Pierre Boulez's 80th birthday with works he composed or was influenced by. The musical ideas in his slow-moving, hypnotic Rituel are carried by long sequences of instrumental and percussive gestures that are repeated with only subtle variations, evoking the ancient Japanese court music known as gagaku. It's written for solo oboe, clarinet duo, flute trio, violin quartet, woodwind quintet, string sextet, woodwind septet, and 14 brass players, and each of these eight configurations is led by a percussionist, often on a non-Western instrument, and positioned around the hall so that the music seems to move around the listener. Also on the program is a work by Boulez's teacher Messiaen, his Sept haikai for piano and small orchestra (1962). It too was inspired by gagaku and has long repeating sections, but it's rhythmically hyperactive and makes vigorous use of the percussion section, including xylophone, marimba, and piano. The pianist will be the phenomenal Pierre-Laurent Aimard, who's long been associated with the music of Messiaen and is one of today's preeminent players of contemporary music--his 2003 performance of Messiaen's Vingt regards sur l'enfant-Jesus was a revelation. Aimard will also play Boulez's First Piano Sonata, a violently passionate work with a lyrical first movement that uses widely spaced notes across the entire range of the piano and short broken chords, gestures, and clusters that defy any attempt to find a meter. And he'll perform Stravinsky's Concerto for Piano and Winds (1923), with its startling opening allegro, its wonderfully sinister second-movement bassoon part and elaborate cadenzas for solo piano, and its thrilling fugal third movement. Concluding the program will be a performance by Reigakusha, Japan's premier gagaku ensemble, whose meditative music combines wooden wind and string instruments and an assortment of percussion, ranging from gentle chimes to blocks and gongs. David Robertson conducts. Fri 3/4 and Sat 3/5, 8 PM, Orchestra Hall, Symphony Center, 220 S. Michigan, 312-294-3000 or 800-223-7114, $19-$110. See also Saturday and Tuesday.