Note: this is just a sampling. Not all presenters publish complete concert program information ahead of time. You may need to phone the organization to determine exactly what compositions will be performed on a given program.
Gotham Early Music--historical performance in New York
The Juilliard School--mostly free, mostly professional-quality student performances
main events page
Historical Performance (early music) events
New York Philharmonic Orchestra
Polyphony--month-by-month listings for concerts of early music
SoundArt--montly listings of new music (contemporary "classical" music) in New York