The month of January was named after the Roman god Janus, patron of transitions and duality, of the intersection between past and present. January’s NSO Showcase program is thus a study in contrasts. Brahms’ “Tragic” Overture is followed by Mozart’s ebullient Eine Kleine Nachtmusik. We’ll hear the late American pianist Nicholas Angelich in his NSO debut, performing Schumann’s romantic Piano Concerto. This eclectic program ends with Shostakovich’s experimental 6th Symphony, a defiant comment on life in Russia during the Stalin years.