Subtitled "Nine Meditations," La Nativité du Seigneur, Olivier Messiaen's first big work for the organ, begins with a slurred cluster of notes. Loose tones stumble along and then pause before a looming chord undergirded by a chirruping note or two. It is one of the rare moments in music where we actually hear a composer pondering where to go next. A crazed clockwork melody follows, winding up only to settle into a serene, chantlike passage that stakes out a sonic world where time is vaporous and to tarry, divine. Don't miss this final installment in the cycle. (St. James Cathedral, 804 Ninth Ave, 382-4874. 7:30 pm, $15 suggested donation.)