This weekend marks Mozart's 234th birthday, and what better way to celebrate than with David Schrader performing (with the City Musick) the Piano Concerto no. 21 in C Major on fortepiano in a new edition by Richard Maunder, the famed musicologist who reconstructed the Mozart Requiem an C Minor Mass for Christopher Hogwood. If your notion of "authentic" Mozart is the dreadful set of British recordings currently being released by Malcolm Bilson and John Eliot Gardiner, then this performance will be a revelation; unlike Bilson, Schrader does not treat the delicate fortepiano as a modern piano, but allows the instrument to be heard on its own terms with its softer, woodwindlike timbres and subtle articulations. But of course, it is Schrader's own unique combination of musicianship and technique, second to none with this music, that will really make this familiar work sound as if you are hearing it for the first time. Mozart piano concerti are piano-orchestra dialogues, and there's probably no conductor out there with a better feel for this music than Elaine Scott Banks, Schrader's collaborator. Also on the all-Mozart program will be the Coronation Mass, the Ave verum corpus, and the Overture to Cosi fan tutte. Saturday, 8 PM, Saint Thomas the Apostle Church, 5472 S. Kimbark. Sunday, 7:30 PM, Pick-Staiger Concert Hall, Northwestern University, 1977 Sheridan, Evanston. 489-2100.