Since 1984, the SEM Ensemble, directed by Petr Kotik, has given annual Christmas concerts. But these are not your usual holiday fare! The programs mix works from the New York School, other pieces in the avant-garde/experimental tradition, and early music. On Tuesday evening December 21 at the Paula Cooper Gallery in Chelsea, SEM will present J. S. Bach’s Fugue in 6 Voices from A Musical Offering (1747), Kotik’s 1st String Quartet (2007-’10), Why Patterns? (1978) by Morton Feldman, and two works by Christian Wolff: Small Preludes (2009-’10) and, incredibly, the American premiere of a work dating from 1958: For Six or Seven Players (for