You can drop-off trees at Greenwood Cemetery at the 5th Avenue & 25th Street Entrance from 8 am to 4:30 pm daily from January 1 thru 9, and until 2 pm on January 10. Also on January 10, from 10 am to 2 pm on Saturday, a tree chipper will be on site and Master Composters from the Brooklyn Compost Project will be available to answer all your composting questions.
The Department of Sanitation will collect trees left at the curb for composting starting Monday, January 5 through Friday, January 16. As always, you must remove all lights, ornaments, and stands from your tree before setting it out at the curb for collection.
I've updated the original post, which has all the details, including a map of chipping and drop-off locations.